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    <headerData>
        <submissionType>144</submissionType>
        <filerInfo>
            <filer>
                <filerCredentials>
                    <cik>0001737231</cik>
                    <ccc>XXXXXXXX</ccc>
                </filerCredentials>
            </filer>
            <liveTestFlag>LIVE</liveTestFlag>



        </filerInfo>
    </headerData>
    <formData>
        <issuerInfo>
            <issuerCik>0000310158</issuerCik>
            <issuerName>MERCK &amp; CO. INC.</issuerName>
            <secFileNumber>001-06571</secFileNumber>
            <issuerAddress>
                <ns2:street1>126 East Lincoln Avenue</ns2:street1>
                <ns2:city>Rahway</ns2:city>
                <ns2:stateOrCountry>NJ</ns2:stateOrCountry>
                <ns2:zipCode>07065</ns2:zipCode>
            </issuerAddress>
            <issuerContactPhone>908-740-4000</issuerContactPhone>
            <nameOfPersonForWhoseAccountTheSecuritiesAreToBeSold>JENNIFER ZACHARY</nameOfPersonForWhoseAccountTheSecuritiesAreToBeSold>
            <relationshipsToIssuer>
                <relationshipToIssuer>Officer</relationshipToIssuer>
            </relationshipsToIssuer>
        </issuerInfo>
        <securitiesInformation>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <brokerOrMarketmakerDetails>
                <name>Morgan Stanley Smith Barney LLC Executive Financial Services</name>
                <address>
                    <ns2:street1>1 New York Plaza</ns2:street1>
                    <ns2:street2>38th Floor</ns2:street2>
                    <ns2:city>New York</ns2:city>
                    <ns2:stateOrCountry>NY</ns2:stateOrCountry>
                    <ns2:zipCode>10004</ns2:zipCode>
                </address>
            </brokerOrMarketmakerDetails>
            <noOfUnitsSold>49499</noOfUnitsSold>
            <aggregateMarketValue>6284779.13</aggregateMarketValue>
            <noOfUnitsOutstanding>2534023084</noOfUnitsOutstanding>
            <approxSaleDate>02/02/2024</approxSaleDate>
            <securitiesExchangeName>NYSE</securitiesExchangeName>
        </securitiesInformation>
        <securitiesToBeSold>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <acquiredDate>01/23/2023</acquiredDate>
            <natureOfAcquisitionTransaction>Restricted stock vesting under a registered plan</natureOfAcquisitionTransaction>
            <nameOfPersonfromWhomAcquired>Issuer</nameOfPersonfromWhomAcquired>
            <isGiftTransaction>N</isGiftTransaction>
            <amountOfSecuritiesAcquired>1235</amountOfSecuritiesAcquired>
            <paymentDate>01/23/2023</paymentDate>
            <natureOfPayment>Not Applicable</natureOfPayment>
        </securitiesToBeSold>
        <securitiesToBeSold>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <acquiredDate>10/12/2022</acquiredDate>
            <natureOfAcquisitionTransaction>Restricted stock vesting under a registered plan</natureOfAcquisitionTransaction>
            <nameOfPersonfromWhomAcquired>Issuer</nameOfPersonfromWhomAcquired>
            <isGiftTransaction>N</isGiftTransaction>
            <amountOfSecuritiesAcquired>410</amountOfSecuritiesAcquired>
            <paymentDate>10/12/2022</paymentDate>
            <natureOfPayment>Not Applicable</natureOfPayment>
        </securitiesToBeSold>
        <securitiesToBeSold>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <acquiredDate>01/10/2023</acquiredDate>
            <natureOfAcquisitionTransaction>Restricted stock vesting under a registered plan</natureOfAcquisitionTransaction>
            <nameOfPersonfromWhomAcquired>Issuer</nameOfPersonfromWhomAcquired>
            <isGiftTransaction>N</isGiftTransaction>
            <amountOfSecuritiesAcquired>248</amountOfSecuritiesAcquired>
            <paymentDate>01/10/2023</paymentDate>
            <natureOfPayment>Not Applicable</natureOfPayment>
        </securitiesToBeSold>
        <securitiesToBeSold>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <acquiredDate>07/13/2022</acquiredDate>
            <natureOfAcquisitionTransaction>Restricted stock vesting under a registered plan</natureOfAcquisitionTransaction>
            <nameOfPersonfromWhomAcquired>Issuer</nameOfPersonfromWhomAcquired>
            <isGiftTransaction>N</isGiftTransaction>
            <amountOfSecuritiesAcquired>394</amountOfSecuritiesAcquired>
            <paymentDate>02/02/2024</paymentDate>
            <natureOfPayment>Not Applicable</natureOfPayment>
        </securitiesToBeSold>
        <securitiesToBeSold>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <acquiredDate>01/23/2023</acquiredDate>
            <natureOfAcquisitionTransaction>Restricted stock vesting under a registered plan</natureOfAcquisitionTransaction>
            <nameOfPersonfromWhomAcquired>Issuer</nameOfPersonfromWhomAcquired>
            <isGiftTransaction>N</isGiftTransaction>
            <amountOfSecuritiesAcquired>14396</amountOfSecuritiesAcquired>
            <paymentDate>01/23/2023</paymentDate>
            <natureOfPayment>Not Applicable</natureOfPayment>
        </securitiesToBeSold>
        <securitiesToBeSold>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <acquiredDate>02/02/2024</acquiredDate>
            <natureOfAcquisitionTransaction>Exercise of options under a registered plan</natureOfAcquisitionTransaction>
            <nameOfPersonfromWhomAcquired>Issuer</nameOfPersonfromWhomAcquired>
            <isGiftTransaction>N</isGiftTransaction>
            <amountOfSecuritiesAcquired>19417</amountOfSecuritiesAcquired>
            <paymentDate>02/02/2024</paymentDate>
            <natureOfPayment>Cash</natureOfPayment>
        </securitiesToBeSold>
        <securitiesToBeSold>
            <securitiesClassTitle>Common</securitiesClassTitle>
            <acquiredDate>02/26/2021</acquiredDate>
            <natureOfAcquisitionTransaction>Restricted stock vesting under a registered plan</natureOfAcquisitionTransaction>
            <nameOfPersonfromWhomAcquired>Issuer</nameOfPersonfromWhomAcquired>
            <isGiftTransaction>N</isGiftTransaction>
            <amountOfSecuritiesAcquired>13399</amountOfSecuritiesAcquired>
            <paymentDate>02/26/2021</paymentDate>
            <natureOfPayment>Not Applicable</natureOfPayment>
        </securitiesToBeSold>
        <nothingToReportFlagOnSecuritiesSoldInPast3Months>Y</nothingToReportFlagOnSecuritiesSoldInPast3Months>
        <noticeSignature>
            <noticeDate>02/02/2024</noticeDate>
            <planAdoptionDates/>
            <signature>/s/ Jennifer Zachary</signature>
        </noticeSignature>
    </formData>
</edgarSubmission>
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144
0001737231
XXXXXXXX
LIVE
0000310158
MERCK & CO. INC.
001-06571
126 East Lincoln Avenue
Rahway
NJ
07065
908-740-4000
JENNIFER ZACHARY
Officer
Common
Morgan Stanley Smith Barney LLC Executive Financial Services
1 New York Plaza
38th Floor
New York
NY
10004
49499
6284779.13
2534023084
02/02/2024
NYSE
Common
01/23/2023
Restricted stock vesting under a registered plan
Issuer
N
1235
01/23/2023
Not Applicable
Common
10/12/2022
Restricted stock vesting under a registered plan
Issuer
N
410
10/12/2022
Not Applicable
Common
01/10/2023
Restricted stock vesting under a registered plan
Issuer
N
248
01/10/2023
Not Applicable
Common
07/13/2022
Restricted stock vesting under a registered plan
Issuer
N
394
02/02/2024
Not Applicable
Common
01/23/2023
Restricted stock vesting under a registered plan
Issuer
N
14396
01/23/2023
Not Applicable
Common
02/02/2024
Exercise of options under a registered plan
Issuer
N
19417
02/02/2024
Cash
Common
02/26/2021
Restricted stock vesting under a registered plan
Issuer
N
13399
02/26/2021
Not Applicable
Y
02/02/2024
/s/ Jennifer Zachary