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                                                                     Exhibit 5.1
                                                                  Conformed Copy



                         [SIMPSON THACHER & BARTLETT]





                                                                  April 27, 2001


American Home Products Corporation
Five Giralda Farms
Madison, New Jersey  07940

Ladies and Gentlemen:

                  We have acted as counsel to American Home Products
Corporation, a Delaware corporation (the "Company"), in connection with the
Registration Statement on Form S-4 (the "Registration Statement") filed by the
Company with the Securities and Exchange Commission (the "Commission") under the
Securities Act of 1933, as amended, relating to the issuance by the Company of
$500,000,000 aggregate principal amount of 5-7/8% Notes due 2004, $1,000,000,000
aggregate principal amount of 6-1/4% Notes due 2006 and $1,500,000,000 aggregate
principal amount of 6.70% Notes due 2011 (collectively, the "Exchange Notes").
The Exchange Notes will be issued under an Indenture, dated as of April 10, 1992
(as amended by the Supplemental Indenture, dated as of October 13, 1992 and as
further amended by the Second Supplemental Indenture, dated as of March 30,
2001, the "Indenture") between the Company and The Chase Manhattan Bank
(successor to Manufacturers Hanover Trust Company), as Trustee. The Exchange
Notes will be offered by the Company in exchange for $500,000,000 aggregate
principal amount of 5-7/8% Notes due 2004, $1,000,000,000 aggregate principal
amount of 6-1/4% Notes due 2006 and $1,500,000,000 aggregate principal amount of
6.70% Notes due 2011 (collectively, the "Notes").

                  We have examined the Registration Statement and the Indenture,
which has been filed with the Commission as an exhibit to the Registration
Statement. We also have examined the originals, or duplicates or certified or
conformed copies, of such records, agreements, instruments and other documents
and have made such other and further investigations as we have deemed relevant
and necessary in connection with the opinions expressed herein. As to questions
of fact material to this opinion, we have relied upon certificates of public
officials and of officers and representatives of the Company.
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                  In rendering the opinion set forth below, we have assumed the
genuineness of all signatures, the legal capacity of natural persons, the
authenticity of all documents submitted to us as originals, the conformity to
original documents of all documents submitted to us as duplicates or certified
or conformed copies, and the authenticity of the originals of such latter
documents. We also have assumed that the Indenture is the valid and legally
binding obligation of the Trustee.

                  Based upon the foregoing, and subject to the qualifications
and limitations stated herein, we are of the opinion that when the Exchange
Notes have been duly executed, authenticated, issued and delivered in accordance
with the provisions of the Indenture upon the exchange, the Exchange Notes will
constitute valid and legally binding obligations of the Company enforceable
against the Company in accordance with their terms.

                  Our opinion set forth above is subject to the effects of (1)
bankruptcy, insolvency, fraudulent conveyance, reorganization, moratorium and
other similar laws relating to or affecting creditors' rights generally, (2)
general equitable principles (whether considered in a proceeding in equity or at
law) and (3) an implied covenant of good faith and fair dealing.

                  We are members of the Bar of the State of New York, and we do
not express any opinion herein concerning any law other than the Delaware
General Corporation Law (including the statutory provisions, all applicable
provisions of the Delaware Constitution and reported judicial decisions
interpreting the foregoing), the law of the State of New York and the Federal
law of the United States.

                  We hereby consent to the filing of this opinion letter as
Exhibit 5.1 to the Registration Statement and to the use of our name under the
caption "Legal Matters" in the Prospectus included in the Registration
Statement.

                                                 Very truly yours,

                                                 /s/ Simpson Thacher & Bartlett
                                                 ------------------------------

                                                 SIMPSON THACHER & BARTLETT

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