
<PAGE>   1

                           OFFER TO PURCHASE FOR CASH

                     ALL OUTSTANDING SHARES OF COMMON STOCK

                                       OF

                            CLINTRIALS RESEARCH INC.

                                       AT

                              $6.00 NET PER SHARE

                                       BY

                            INDIGO ACQUISITION CORP.
                           A WHOLLY OWNED SUBSIDIARY

                                       OF

                        INVERESK RESEARCH (CANADA) INC.
                           A WHOLLY OWNED SUBSIDIARY

                                       OF

                        INVERESK RESEARCH GROUP LIMITED

  THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE AT 12:00 MIDNIGHT, NEW YORK CITY
         TIME, ON MONDAY, APRIL 2, 2001, UNLESS THE OFFER IS EXTENDED.

                                                                   March 5, 2001

To Our Clients:

     Enclosed for your consideration are the Offer to Purchase dated March 5,
2001 (the "Offer to Purchase") and the related Letter of Transmittal (which,
together with any amendments or supplements thereto, collectively constitute the
"Offer") in connection with the offer by Indigo Acquisition Corp., a Delaware
corporation ("Purchaser"), which is a wholly owned subsidiary of Inveresk
Research (Canada) Inc., a corporation organized under the laws of Canada
("Inveresk Canada"), which, in turn, is a wholly owned subsidiary of Inveresk
Research Group Limited ("Parent"), a company organized under the laws of
Scotland, to purchase all outstanding shares of common stock, par value $0.01
per share (the "Shares"), of ClinTrials Research Inc., a Delaware corporation
(the "Company"), at $6.00 per Share, net to the seller in cash, upon the terms
and subject to the conditions set forth in the Offer to Purchase. Also enclosed
is the Letter to Stockholders of the Company from Paul Ottaviano, President and
Chief Executive Officer of the Company, accompanied by the Company's
Solicitation/Recommendation Statement on Schedule 14D-9.

     WE (OR OUR NOMINEES) ARE THE HOLDER OF RECORD OF SHARES HELD FOR YOUR
ACCOUNT. A TENDER OF SUCH SHARES CAN BE MADE ONLY BY US AS THE HOLDER OF RECORD
AND PURSUANT TO YOUR INSTRUCTIONS. THE ENCLOSED LETTER OF TRANSMITTAL IS
FURNISHED TO YOU FOR YOUR INFORMATION ONLY AND CANNOT BE USED BY YOU TO TENDER
SHARES HELD BY US FOR YOUR ACCOUNT.

     We request instructions as to whether you wish us to tender any or all of
the Shares held by us for your account, upon the terms and subject to the
conditions set forth in the Offer. Your attention is invited to the following:

          1. The tender price is $6.00 per Share, net to you in cash, without
     interest thereon, upon the terms and subject to the conditions set forth in
     the Offer.

          2. The Offer is being made for all outstanding Shares.
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          3. This Offer is being made pursuant to an Agreement and Plan of
     Merger (the "Merger Agreement") dated as of February 22, 2001, by and among
     Parent, Purchaser and the Company. The Board of Directors of the Company,
     at a meeting held on February 22, 2001, by unanimous vote determined that
     the terms of the Offer and the Merger are fair to, and in the best
     interests of, the Company and the Company's stockholders, approved the
     Merger and the other transactions contemplated by the Merger Agreement and
     approved the Merger Agreement. The Board of Directors unanimously
     recommends that the Company's stockholders accept the Offer, tender their
     Shares in the Offer and, if required under Delaware law or the Company's
     Certificate of Incorporation or Bylaws, vote to adopt the Merger Agreement.

          4. THE OFFER AND WITHDRAWAL RIGHTS EXPIRE AT 12:00 MIDNIGHT, NEW YORK
     CITY TIME, ON MONDAY, APRIL 2, 2001, UNLESS THE PURCHASER EXTENDS THE TIME
     DURING WHICH THE OFFER IS OPEN. IF ON THE EXPIRATION OF THE OFFER THE
     NUMBER OF SHARES THAT HAVE BEEN PROPERLY TENDERED AND NOT SUBSEQUENTLY
     WITHDRAWN REPRESENTS MORE THAN 50% BUT LESS THAN 90% OF THE OUTSTANDING
     SHARES (CALCULATED ON A FULLY DILUTED BASIS), PURCHASER INTENDS TO ELECT TO
     PROVIDE A SUBSEQUENT OFFERING PERIOD PURSUANT TO RULE 14d-11 UNDER THE
     SECURITIES EXCHANGE ACT OF 1934, AS AMENDED.

          5. THE OFFER IS CONDITIONED UPON, AMONG OTHER THINGS, (A) A NUMBER OF
     SHARES BEING VALIDLY TENDERED AND NOT WITHDRAWN ON THE APPLICABLE
     EXPIRATION DATE OF THE OFFER THAT, TOGETHER WITH ANY SHARES OWNED BY PARENT
     OR ANY OF ITS AFFILIATES (INCLUDING PURCHASER), REPRESENTS AT LEAST A
     MAJORITY OF THE TOTAL NUMBER OF ALL OUTSTANDING SHARES PLUS ALL SHARES
     ISSUABLE UPON EXERCISE OF OPTIONS AND OTHER SIMILAR RIGHTS TO PURCHASE
     SHARES AND (B) THE RECEIPT OF APPROVALS REQUIRED BY OR THE EXPIRATION OR
     TERMINATION OF THE APPLICABLE WAITING PERIODS UNDER UNITED STATES AND
     EUROPEAN ANTITRUST AND COMPETITION LAWS. THE OFFER IS ALSO SUBJECT TO THE
     SATISFACTION OR WAIVER OF CERTAIN OTHER CONDITIONS. SEE SECTIONS 1 AND 13
     OF THE OFFER TO PURCHASE.

          6. Any stock transfer taxes applicable to the sale of the Shares to
     Purchaser pursuant to the Offer will be paid by Purchaser, except as
     otherwise provided in Instruction 6 of the Letter of Transmittal.

          7. Stockholders of record who tender Shares directly will not be
     obligated to pay brokerage fees or commissions to the Dealer Manager, the
     Depositary or the Information Agent or, except as set forth in Instruction
     6 of the Letter of Transmittal, stock transfer taxes on the purchase of
     Shares by Purchaser pursuant to the Offer. However, Federal income tax
     backup withholding at a rate of 31% may be required, unless an exemption is
     provided or unless the required taxpayer identification information is
     provided. See Instruction 8 of the Letter of Transmittal.

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     If you wish to have us tender any or all of your Shares, please so instruct
us by completing, executing and returning to us the instruction form set forth
on the reverse side of this letter. An envelope to return your instructions to
us is enclosed. If you authorize the tender of your Shares, all such Shares will
be tendered unless otherwise specified on the reverse side of this letter. YOUR
INSTRUCTIONS SHOULD BE FORWARDED TO US IN AMPLE TIME TO PERMIT US TO SUBMIT A
TENDER ON YOUR BEHALF PRIOR TO THE EXPIRATION OF THE OFFER.

     In all cases payment for Shares accepted for payment pursuant to the Offer
will be made only after timely receipt by SunTrust Bank (the "Depositary") of
(i) certificates for those Shares, or timely confirmation of a book-entry
transfer of those Shares into the Depositary's account at the Depository Trust
Company, pursuant to the procedures described in Section 3 of the Offer to
Purchase, (ii) the Letter of Transmittal or a manually signed facsimile of the
Letter of Transmittal, properly completed and duly executed, with any required
signature guarantees, or, in the case of a book-entry transfer, an Agent's
Message (as defined in the Offer to Purchase) and (iii) any other documents
required by the Letter of Transmittal. Accordingly, tendering stockholders may
be paid at different times depending upon when certificates for Shares or
Book-Entry Confirmations (as defined in the Offer to Purchase) with respect to
Shares are actually received by the Depositary. UNDER NO CIRCUMSTANCES WILL
PURCHASER PAY INTEREST ON THE PURCHASE PRICE OF THE SHARES TO BE PAID BY THE
PURCHASER, REGARDLESS OF ANY EXTENSION OF THE OFFER OR ANY DELAY IN PAYING FOR
SHARES.

     The Offer is being made solely by the Offer to Purchase and the related
Letter of Transmittal and is being made to all holders of the Shares. Purchaser
is not aware of any state where the making of the Offer is prohibited by
administrative or judicial action pursuant to any valid state statute. If
Purchaser becomes aware of any valid state statute prohibiting the making of the
Offer or the acceptance of the Shares pursuant thereto, Purchaser shall make a
good faith effort to comply with such statute or seek to have such statute
declared inapplicable to the Offer. If, after such good faith effort, Purchaser
cannot comply with such state statute, the Offer will not be made to (nor will
tenders be accepted from or on behalf of) holders of the Shares in such state.
In those jurisdictions where the blue sky or other laws require the Offer to be
made by a licensed broker or dealer, the Offer is being made on behalf of
Purchaser by the Dealer Manager or one or more registered brokers or dealers
licensed under the laws of such jurisdictions.

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<PAGE>   4

                        INSTRUCTIONS WITH RESPECT TO THE
                           OFFER TO PURCHASE FOR CASH
                     ALL OUTSTANDING SHARES OF COMMON STOCK

                                       OF

                            CLINTRIALS RESEARCH INC.

                                       BY

                            INDIGO ACQUISITION CORP.
                          A WHOLLY OWNED SUBSIDIARY OF

                        INVERESK RESEARCH (CANADA) INC.
                          A WHOLLY OWNED SUBSIDIARY OF

                        INVERESK RESEARCH GROUP LIMITED

     The undersigned acknowledge(s) receipt of your letter and the enclosed
Offer to Purchase dated March 5, 2001 (the "Offer to Purchase"), and the related
Letter of Transmittal (which, together with any amendments or supplements
thereto, collectively constitute the "Offer") in connection with the Offer by
Indigo Acquisition Corp., a Delaware corporation, which is a wholly owned
subsidiary of Inveresk Research (Canada) Inc., a corporation organized under the
laws of Canada ("Inveresk Canada"), which in turn is a wholly owned subsidiary
of Inveresk Research Group Limited, a company organized under the laws of
Scotland ("Parent"), to purchase all outstanding shares of common stock, par
value $0.01 per share (the "Shares"), of ClinTrials Research Inc., a Delaware
corporation (the "Company"), at $6.00 per Share, net to the seller in cash, upon
the terms and subject to the conditions set forth in the Offer.

     This will instruct you to tender to Purchaser the number of Shares
indicated below (or if no number is indicated below, all Shares) held by you for
the account of the undersigned, upon the terms and subject to the conditions set
forth in the Offer.

   Number of Shares to be tendered:*
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   Certificate Nos. (if available):
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   Account No.:
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   Dated:
   ------------------------------ , 2001

                                   SIGN HERE

   Signature(s):
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   Please type or print Name(s):
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   Please type or print address(es):
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   Area Code and Telephone Number: (   )

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   Taxpayer Identification or Social Security Number(s):
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* Unless otherwise indicated, it will be assumed that all Shares held by us for
  your account are to be tendered.
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