-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
Originator-Key-Asymmetric:
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 FTSdNbrLJQZ6F60cWvGlCHL7wseWe+BAyZ9p8zIfHyoNGQ+ZvxTx0kyAaH2FWHuV
 unPGAmjUU3K/FKaxNSo80g==

<SEC-DOCUMENT>0001463229-10-000006.txt : 20100319
<SEC-HEADER>0001463229-10-000006.hdr.sgml : 20100319
<ACCEPTANCE-DATETIME>20100319153820
ACCESSION NUMBER:		0001463229-10-000006
CONFORMED SUBMISSION TYPE:	D/A
PUBLIC DOCUMENT COUNT:		1
ITEM INFORMATION:		Rule 506
ITEM INFORMATION:		Investment Company Act Section 3(c)
ITEM INFORMATION:		Section 3(c)(7)
FILED AS OF DATE:		20100319
DATE AS OF CHANGE:		20100319
EFFECTIVENESS DATE:		20100319

FILER:

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			GMO MEAN REVERSION FUND (ONSHORE), A SERIES OF GMO MASTER PORTFOLIOS (ONSHORE), L.P.
		CENTRAL INDEX KEY:			0001246099
		IRS NUMBER:				000000000
		STATE OF INCORPORATION:			DE

	FILING VALUES:
		FORM TYPE:		D/A
		SEC ACT:		1933 Act
		SEC FILE NUMBER:	021-56376
		FILM NUMBER:		10694180

	BUSINESS ADDRESS:	
		STREET 1:		40 ROWES WHARF
		CITY:			BOSTON
		STATE:			MA
		ZIP:			02110
		BUSINESS PHONE:		617-330-7500

	MAIL ADDRESS:	
		STREET 1:		40 ROWES WHARF
		CITY:			BOSTON
		STATE:			MA
		ZIP:			02110

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	GMO MEAN REVERSION FUND ONSHORE LP
		DATE OF NAME CHANGE:	20030624
</SEC-HEADER>
<DOCUMENT>
<TYPE>D/A
<SEQUENCE>1
<FILENAME>primary_doc.xml
<TEXT>
<XML>
<?xml version="1.0"?>
<edgarSubmission>

    <schemaVersion>X0603</schemaVersion>

    <submissionType>D/A</submissionType>

    <testOrLive>LIVE</testOrLive>

    <primaryIssuer>
        <cik>0001246099</cik>
        <entityName>GMO MEAN REVERSION FUND (ONSHORE), A SERIES OF GMO MASTER PORTFOLIOS (ONSHORE), L.P.</entityName>
        <issuerAddress>
            <street1>40 ROWES WHARF</street1>
            <city>BOSTON</city>
            <stateOrCountry>MA</stateOrCountry>
            <zipCode>02110</zipCode>
        </issuerAddress>
        <issuerPhoneNumber>617-330-7500</issuerPhoneNumber>
        <jurisdictionOfInc>DELAWARE</jurisdictionOfInc>
        <edgarPreviousNameList>
            <previousName>GMO MEAN REVERSION FUND ONSHORE LP</previousName>
        </edgarPreviousNameList>
        <entityType>Limited Partnership</entityType>
        <yearOfInc>
            <overFiveYears>true</overFiveYears>
        </yearOfInc>
    </primaryIssuer>

    <relatedPersonsList>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>MARC</firstName>
                <middleName>O.</middleName>
                <lastName>MAYER</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>C/O GMO INVESTMENT PARTNERS, LLC</street1>
                <street2>40 ROWES WHARF</street2>
                <city>BOSTON</city>
                <stateOrCountry>MA</stateOrCountry>
                <zipCode>02110</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>SHEP</firstName>
                <lastName>BURNETT</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>C/O GMO INVESTMENT PARTNERS, LLC</street1>
                <street2>40 ROWES WHARF</street2>
                <city>BOSTON</city>
                <stateOrCountry>MA</stateOrCountry>
                <zipCode>02110</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>GREGORY</firstName>
                <middleName>L.</middleName>
                <lastName>POTTLE</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>C/O GMO INVESTMENT PARTNERS, LLC</street1>
                <street2>40 ROWES WHARF</street2>
                <city>BOSTON</city>
                <stateOrCountry>MA</stateOrCountry>
                <zipCode>02110</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>DAVID</firstName>
                <lastName>BOHAN</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>C/O GMO INVESTMENT PARTNERS, LLC</street1>
                <street2>40 ROWES WHARF</street2>
                <city>BOSTON</city>
                <stateOrCountry>MA</stateOrCountry>
                <zipCode>02110</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
    </relatedPersonsList>

    <offeringData>
        <industryGroup>
            <industryGroupType>Pooled Investment Fund</industryGroupType>
            <investmentFundInfo>
                <investmentFundType>Hedge Fund</investmentFundType>
                <is40Act>true</is40Act>
            </investmentFundInfo>
        </industryGroup>
        <issuerSize>
            <aggregateNetAssetValueRange>Decline to Disclose</aggregateNetAssetValueRange>
        </issuerSize>
        <federalExemptionsExclusions>
            <item>06</item>
            <item>3C</item>
            <item>3C.7</item>
        </federalExemptionsExclusions>
        <typeOfFiling>
            <newOrAmendment>
                <isAmendment>true</isAmendment>
                <previousAccessionNumber>9999999997-09-014707</previousAccessionNumber>
            </newOrAmendment>
            <dateOfFirstSale>
                <value>2003-02-28</value>
            </dateOfFirstSale>
        </typeOfFiling>
        <durationOfOffering>
            <moreThanOneYear>true</moreThanOneYear>
        </durationOfOffering>
        <typesOfSecuritiesOffered>
            <isPooledInvestmentFundType>true</isPooledInvestmentFundType>
        </typesOfSecuritiesOffered>
        <businessCombinationTransaction>
            <isBusinessCombinationTransaction>false</isBusinessCombinationTransaction>
            <clarificationOfResponse></clarificationOfResponse>
        </businessCombinationTransaction>
        <minimumInvestmentAccepted>0</minimumInvestmentAccepted>
        <salesCompensationList></salesCompensationList>
        <offeringSalesAmounts>
            <totalOfferingAmount>Indefinite</totalOfferingAmount>
            <totalAmountSold>210542531</totalAmountSold>
            <totalRemaining>Indefinite</totalRemaining>
            <clarificationOfResponse></clarificationOfResponse>
        </offeringSalesAmounts>
        <investors>
            <hasNonAccreditedInvestors>false</hasNonAccreditedInvestors>
            <totalNumberAlreadyInvested>47</totalNumberAlreadyInvested>
        </investors>
        <salesCommissionsFindersFees>
            <salesCommissions>
                <dollarAmount>0</dollarAmount>
            </salesCommissions>
            <findersFees>
                <dollarAmount>0</dollarAmount>
            </findersFees>
            <clarificationOfResponse></clarificationOfResponse>
        </salesCommissionsFindersFees>
        <useOfProceeds>
            <grossProceedsUsed>
                <dollarAmount>0</dollarAmount>
            </grossProceedsUsed>
            <clarificationOfResponse></clarificationOfResponse>
        </useOfProceeds>
        <signatureBlock>
            <authorizedRepresentative>false</authorizedRepresentative>
            <signature>
                <issuerName>GMO MEAN REVERSION FUND (ONSHORE), A SERIES OF GMO MASTER PORTFOLIOS (ONSHORE), L.P.</issuerName>
                <signatureName>DAVID BOHAN</signatureName>
                <nameOfSigner>DAVID BOHAN</nameOfSigner>
                <signatureTitle>VICE-PRESIDENT OF THE GENERAL PARTNER OF THE ISSUER</signatureTitle>
                <signatureDate>2010-03-19</signatureDate>
            </signature>
        </signatureBlock>
    </offeringData>
</edgarSubmission>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
-----END PRIVACY-ENHANCED MESSAGE-----