
<PAGE>   1

                                UNITED STATES
                      SECURITIES AND EXCHANGE COMMISSION
                            WASHINGTON, D.C. 20549


                                 SCHEDULE 13G


                  UNDER THE SECURITIES EXCHANGE ACT OF 1934

                             (AMENDMENT NO. 1 )*


                           Clintrials Research Inc.
- ------------------------------------------------------------------------------
                               (Name of Issuer)

                                 Common Stock
- ------------------------------------------------------------------------------
                        (Title of Class of Securities)

                                  188767107
- ------------------------------------------------------------------------------
                                (CUSIP Number)

           December 31, 2000 - Filing pursuant to Rule 13d-1(b)(2)
- ------------------------------------------------------------------------------
           (Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule
is filed:

         [x] Rule 13d-1(b)
         [ ] Rule 13d-1(c)
         [ ] Rule 13d-1(d)

* The remainder of this cover page shall be filled out for a reporting
person's initial filing on this form with respect to the subject class of
securities, and for any subsequent amendment containing information which
would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be
deemed to be "filed" for the purpose of Section 18 of the Securities Exchange
Act of 1934 ("Act") or otherwise subject to the liabilities of that section of
the Act but shall be subject to all other provisions of the Act (however, see
the Notes).



<PAGE>   2


CUSIP NO. 188767107


<TABLE>
<S>                                                                                                  <C>
- --------------------------------------------------------------------------------------------------------
   1.      NAMES OF REPORTING PERSONS.
           I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY).

           Longwood Investment Advisors, Inc.
           23-2745059
- --------------------------------------------------------------------------------------------------------
   2.      CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)
                                                                                  (a)  [  ]
                                                                                  (b)  [x ]
- --------------------------------------------------------------------------------------------------------
   3.      SEC USE ONLY


- --------------------------------------------------------------------------------------------------------
   4.      CITIZENSHIP OR PLACE OF ORGANIZATION

           Pennsylvania
- --------------------------------------------------------------------------------------------------------
                     5.     SOLE VOTING POWER

                             -0-
  NUMBER OF        -------------------------------------------------------------------------------------
    SHARES           6.     SHARED VOTING POWER
 BENEFICIALLY
   OWNED BY                 1,620,918
     EACH          -------------------------------------------------------------------------------------
  REPORTING          7.     SOLE DISPOSITIVE POWER
    PERSON
    WITH:                   -0-
                   -------------------------------------------------------------------------------------
                     8.     SHARED DISPOSITIVE POWER

                            1,620,918
- --------------------------------------------------------------------------------------------------------
   9.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

           1,620,918

- --------------------------------------------------------------------------------------------------------
   10.     CHECK IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS)


           N/A
- --------------------------------------------------------------------------------------------------------
   11.     PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)

           8.8%
- ---------- ---------------------------------------------------------------------------------------------
   12.     TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)

           IA
- --------------------------------------------------------------------------------------------------------
</TABLE>



<PAGE>   3



CUSIP NO. 188767107


<TABLE>
<S>                                                                                                  <C>
- --------------------------------------------------------------------------------------------------------
   1.      NAMES OF REPORTING PERSONS.
           I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY).


           Robert A. Davidson
- --------------------------------------------------------------------------------------------------------
   2.      CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)
                                                                                  (a)  [  ]
                                                                                  (b)  [x ]
- --------------------------------------------------------------------------------------------------------
   3.      SEC USE ONLY


- --------------------------------------------------------------------------------------------------------
   4.      CITIZENSHIP OR PLACE OF ORGANIZATION

           United States
- --------------------------------------------------------------------------------------------------------
                     5.     SOLE VOTING POWER

                            96,000
 NUMBER OF         -------------------------------------------------------------------------------------
   SHARES            6.     SHARED VOTING POWER
BENEFICIALLY
  OWNED BY                  1,620,918
    EACH           -------------------------------------------------------------------------------------
 REPORTING           7.     SOLE DISPOSITIVE POWER
   PERSON
   WITH:                    96,000
                   -------------------------------------------------------------------------------------
                     8.     SHARED DISPOSITIVE POWER

                            1,620,918
- --------------------------------------------------------------------------------------------------------
   9.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON


           1,716,918
- --------------------------------------------------------------------------------------------------------
   10.     CHECK IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS)


           N/A
- --------------------------------------------------------------------------------------------------------
   11.     PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)

           9.3%
- --------------------------------------------------------------------------------------------------------
   12.     TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)

           IN
- --------------------------------------------------------------------------------------------------------
</TABLE>



<PAGE>   4
CUSIP NO. 188767107

<TABLE>
<S>                                                                                                  <C>
- --------------------------------------------------------------------------------------------------------
   1.      NAMES OF REPORTING PERSONS.
           I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (ENTITIES ONLY).

           John P. McNiff
- --------------------------------------------------------------------------------------------------------
   2.      CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (SEE INSTRUCTIONS)
                                                                                  (a)  [  ]
                                                                                  (b)  [x ]
- --------------------------------------------------------------------------------------------------------
   3.      SEC USE ONLY


- --------------------------------------------------------------------------------------------------------
   4.      CITIZENSHIP OR PLACE OF ORGANIZATION

           United States
- --------------------------------------------------------------------------------------------------------
                     5.     SOLE VOTING POWER

                             -0-
 NUMBER OF         -------------------------------------------------------------------------------------
   SHARES            6.     SHARED VOTING POWER
BENEFICIALLY
  OWNED BY                  1,620,918
    EACH           -------------------------------------------------------------------------------------
 REPORTING           7.     SOLE DISPOSITIVE POWER
   PERSON
   WITH:                    -0-
                   -------------------------------------------------------------------------------------
                     8.     SHARED DISPOSITIVE POWER

                            1,620,918
- --------------------------------------------------------------------------------------------------------
   9.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

           1,620,918
- --------------------------------------------------------------------------------------------------------
   10.     CHECK IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES (SEE INSTRUCTIONS)


           N/A
- --------------------------------------------------------------------------------------------------------
   11.     PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)

           8.8%
- --------------------------------------------------------------------------------------------------------
   12.     TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)

           IN
- --------------------------------------------------------------------------------------------------------
</TABLE>

<PAGE>   5


Item 1.           (a)      Name of Issuer:

                           Clintrials Research Inc. (the "Issuer")

                  (b)      Address of Issuer's Principal Executive Offices:

                           11000 Weston Parkway, Cary, NC 27513

Item 2.           (a)      Names of Persons Filing:

                           This statement is filed on behalf of (i) Longwood
                           Investment Advisors, Inc., a Pennsylvania
                           corporation ("LIA"), (ii) Robert A. Davidson and
                           (iii) John P. McNiff (the "Reporting Persons"). LIA
                           is registered as an investment adviser under the
                           Investment Advisers Act of 1940, as amended. Robert
                           A. Davidson is Chief Investment Officer and a
                           Managing Director of LIA and is responsible for the
                           selection, acquisition and disposition of
                           securities on behalf of LIA. John P. McNiff is
                           President and a Managing Director of LIA and, as
                           such, shares responsibility with Mr. Davidson for
                           selection, acquisition and disposition of
                           securities on behalf of LIA.

                  (b)      Address of Principal Business Office or, if None,
                           Residence:

                           The address of LIA, Mr. Davidson and Mr. McNiff is:
                           Three Radnor Corporate Center, Suite 300, 100
                           Matsonford Road, Radnor, Pennsylvania 19087.

                  (c)      Citizenship:

                           LIA is a Pennsylvania corporation.  Mr. Davidson
                           and Mr. McNiff are United States citizens.

                  (d)      Title of Class of Securities: Common Stock

                  (e)      CUSIP Number:  188767107

Item 3.           If this statement is filed pursuant to Sections 240.13d-1(b)
                  or 240.13d-2(b) or (c), check whether the person filing is
                  a:

                  (a)      [  ]     Broker or dealer registered under section
                                    15 of the Act (15 U.S.C. 78o).

                  (b)      [  ]     Bank as defined in section 3(a)(6) of the
                                    Act (15 U.S.C. 78c).

                  (c)      [  ]     Insurance company as defined in section
                                    3(a)(19) of the Act (15 U.S.C. 78c).

                  (d)      [  ]     Investment company registered under
                                    section 8 of the Investment Company
                                    Act of 1940 (15 U.S.C. 80a-8).

                  (e)      [x ]     An investment adviser in accordance with
                                    Section 240.13d-1(b)(1)(ii)(E).

                  (f)      [  ]     An employee benefit plan or endowment fund
                                    in accordance with Section
                                    240.13d-1(b)(1)(ii)(F).

                  (g)      [  ]     A parent holding company or control person
                                    in accordance with Section
                                    240.13d-1(b)(1)(ii)(G).

                  (h)      [  ]     A savings association as defined in
                                    section 3(b) of the Federal Deposit
                                    Insurance Act (12 U.S.C. 1813).

                  (i)      [  ]     A church plan that is excluded from the
                                    definition of an investment company
                                    under section 3(c)(14) of the Investment
                                    Company Act of 1940 (15 U.S.C. 80a-3).

                  (j)      [  ]     Group, in accordance with Section
                                    240.13d-1(b)(1)(ii) (J).


<PAGE>   6


Item 4.           Ownership.

                  Provide the following information regarding the aggregate
                  number and percentage of the class of securities of the
                  Issuer identified in Item 1.

                  (a)      Amount beneficially owned:

                           LIA has shared power to vote or direct the vote,
                           and shared power to dispose or direct the
                           disposition of, 1,620,918 shares of the issuer's
                           common stock. Robert A. Davidson and John P. McNiff
                           have shared power to dispose or direct the
                           disposition of 1,620,918 shares of common stock by
                           virtue of being responsible for the selection,
                           acquisition and disposition of the portfolio
                           securities on behalf of LIA. Mr. Davidson has sole
                           power to vote or direct the vote, and sole power to
                           dispose or direct the disposition of 96,000 shares
                           of the issuer's common stock.

                  (b)      Percent of class: 8.8% for LIA and Mr. McNiff; 9.3%
                           for Mr. Davidson

                  (c)      Number of shares as to which the person has:

                           (i)      Sole power to vote or to direct the vote:
                                    0 shares for LIA and Mr. McNiff; 96,000
                                    shares for Mr. Davidson

                           (ii)     Shared power to vote or to direct the
                                    vote: 1,620,918 shares for all Reporting
                                    Persons

                           (iii)    Sole power to dispose or to direct the
                                    disposition of: 0 shares for LIA and Mr.
                                    McNiff; 96,000 shares for Mr. Davidson

                           (iv)     Shared power to dispose or to direct the
                                    disposition of: 1,620,918 shares for all
                                    Reporting Persons

                  Instruction.  For computations regarding securities which
                  represent a right to acquire an underlying security see
                  Section 240.13d-3(d)(1).

Item 5.           Ownership of Five Percent or Less of a Class.

                  If this statement is being filed to report the fact that as
                  of the date hereof the reporting person has ceased to be the
                  beneficial owner of more than five percent of the class of
                  securities, check the following [ ].

                  Instruction:  Dissolution of a group requires a response to
                  this item.

Item 6.           Ownership of More than Five Percent on Behalf of Another
                  Person.

                  If any other person is known to have the right to receive or
                  the power to direct the receipt of dividends from, or the
                  proceeds from the sale of, such securities, a statement to
                  that effect should be included in response to this item and,
                  if such interest relates to more than five percent of the
                  class, such person should be identified. A listing of the

<PAGE>   7

                  shareholders of an investment company registered under the
                  Investment Company Act of 1940 or the beneficiaries of
                  employee benefit plan, pension fund or endowment fund is not
                  required.

                  Except with respect to the 96,000 shares owned by Mr.
                  Davidson, other persons have the right to receive, or the
                  power to direct, the receipt of dividends from, or the
                  proceeds from the sale of, the issuer's shares which are the
                  subject of this Amendment to Schedule 13G. Of such shares,
                  Longwood Partners, L.P., of which LIA is the investment
                  advisor, has such right with respect to 372,727 shares.

Item 7.           Identification and Classification of the Subsidiary Which
                  Acquired the Security Being Reported on By the Parent
                  Holding Company or Control Person.

                  If a parent holding company or control person has filed this
                  schedule, pursuant to Rule 13d-1(b)(1)(ii)(G), so indicate
                  under Item 3(g) and attach an exhibit stating the identity
                  and the Item 3 classification of the relevant subsidiary. If
                  a parent holding company or control person has filed this
                  schedule pursuant to Rule 13d-1(c) or Rule 13d-1(d), attach
                  an exhibit stating the identification of the relevant
                  subsidiary.

                  Not applicable.

Item 8.           Identification and Classification of Members of the Group.

                  If a group has filed this schedule pursuant to Section
                  240.13d-1(b)(1)(ii)(J), so indicate under Item 3(j) and
                  attach an exhibit stating the identity and Item 3
                  classification of each member of the group. If a group has
                  filed this schedule pursuant to Rule 240.13d-1(c) or Rule
                  240.13d-1(d), attach an exhibit stating the identity of each
                  member of the group.

                  Not applicable.

Item 9.           Notice of Dissolution of Group.

                  Notice of dissolution of a group may be furnished as an
                  exhibit stating the date of the dissolution and that all
                  further filings with respect to transactions in the security
                  reported on will be filed, if required, by members of the
                  group, in their individual capacity. See Item 5.

                  Not applicable.

Item 10.                Certification.

                  By signing below we certify that, to the best of our
                  knowledge and belief, the securities referred to above were
                  acquired and are held in the ordinary course of business and
                  were not acquired and are not held for the purpose of or
                  with the effect of changing or influencing the control of
                  the issuer of the securities and were not acquired and are
                  not held in connection with or as a participant in any
                  transaction having that purpose or effect.

<PAGE>   8


                                  SIGNATURE


         After reasonable inquiry and to the best of our knowledge and belief,
we certify that the information set forth in this statement is true, complete
and correct. We also hereby agree to file this statement jointly pursuant to
the agreement set forth as Exhibit 1 hereto.

Dated:   May 2, 2001


LONGWOOD INVESTMENT ADVISORS

By:      /s/ Robert A. Davidson
         ----------------------
         Robert A. Davidson
         Chief Investment Officer

By:      /s/ John P. McNiff
         ------------------
         John P. McNiff
         President


         /s/ Robert A. Davidson
         ----------------------
         Robert A. Davidson


         /s/ John P. McNiff
         ------------------
         John P. McNiff










