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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
 HozkK9tfdbNemKuXNVBRHRBsbz3KoZKtKmSxg1ISMMJ+jn8cuRYnsnvVQdrqhPCJ
 0PP9/Hai0g9uvRDJNOl30w==

<SEC-DOCUMENT>0001140361-10-013713.txt : 20100326
<SEC-HEADER>0001140361-10-013713.hdr.sgml : 20100326
<ACCEPTANCE-DATETIME>20100326105710
ACCESSION NUMBER:		0001140361-10-013713
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20100325
FILED AS OF DATE:		20100326
DATE AS OF CHANGE:		20100326

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			PORTNOY ADAM
		CENTRAL INDEX KEY:			0001273871

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-15319
		FILM NUMBER:		10706522

	MAIL ADDRESS:	
		STREET 1:		C/O REIT MANAGEMENT & RESEARCH
		STREET 2:		400 CENTRE ST
		CITY:			NEWTON
		STATE:			MA
		ZIP:			02458

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			SENIOR HOUSING PROPERTIES TRUST
		CENTRAL INDEX KEY:			0001075415
		STANDARD INDUSTRIAL CLASSIFICATION:	REAL ESTATE INVESTMENT TRUSTS [6798]
		IRS NUMBER:				043445278
		STATE OF INCORPORATION:			MD
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		400 CENTRE STREET
		CITY:			NEWTON
		STATE:			MA
		ZIP:			02458
		BUSINESS PHONE:		6173323990
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>doc1.xml
<DESCRIPTION>FORM 4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-03-25</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001075415</issuerCik>
        <issuerName>SENIOR HOUSING PROPERTIES TRUST</issuerName>
        <issuerTradingSymbol>NYSE: SNH</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001273871</rptOwnerCik>
            <rptOwnerName>PORTNOY ADAM</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O REIT MANAGEMENT &amp; RESEARCH LLC</rptOwnerStreet1>
            <rptOwnerStreet2>400 CENTRE STREET</rptOwnerStreet2>
            <rptOwnerCity>NEWTON</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02458</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Shares of Beneficial Interest</value>
            </securityTitle>
            <transactionDate>
                <value>2010-03-25</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>11314</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>22.15</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>96756.55</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">These shares were issued pursuant to a business management agreement between the issuer and Reit Management &amp; Research LLC (&quot;RMR&quot;) of which Mr. Portnoy is a director and minority beneficial owner.  Mr. Portnoy purchased these shares from RMR upon their issuance. Such transfers were exempt from Section 16 of the Securities Exchange Act of 1934, as amended, pursuant to Rule 16a-13 thereunder.</footnote>
        <footnote id="F2">In accordance with the business management agreement referred to above, certain historical share price information was used to determine the number of common shares of benefical interest paid to RMR.  The per share price amount listed above is the closing sale price of the issuer's common shares of beneficial interest on the New York Stock Exchange on March 25, 2010, when the shares were issued and purchased.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Adam D. Portnoy</signatureName>
        <signatureDate>2010-03-25</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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