Exhibit 5.1
Conformed Copy

S  i  m  p  s  o  n   T  h  a  c  h  e  r   &   B  a  r  t  l  e  t  t    l   l   p

425 Lexington Avenue

New York, N.Y. 10017-3954

(212) 455-2000

Facsimile (212) 455-2502

December 16, 2005

Wyeth
Five Giralda Farms
Madison, New Jersey 07940

Ladies and Gentlemen:

     We have acted as counsel to Wyeth, 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 $1,000,000,000 aggregate principal amount of 5¹/2% Notes due 2016 and $500,000,000 aggregate principal amount of 6% Notes due 2036 (collectively, the “Exchange Securities”). The Exchange Securities 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 Sixth Supplemental Indenture dated as of November 14, 2005, the “Indenture”) between the Company and JPMorgan Chase Bank, N.A. (as successor to Manufacturers Hanover Trust Company), as trustee (the “Trustee”). The Exchange Securities will be offered by the Company in exchange for $1,000,000,000 aggregate principal amount of its outstanding 5¹/2% Notes due 2016 and $500,000,000 aggregate principal amount of its outstanding 6% Notes due 2036.

     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

Wyeth -2- December 16, 2005

originals, or duplicates or certified or conformed copies, of such corporate records, agreements, documents and other instruments and have made such other investigations as we have deemed relevant and necessary in connection with the opinion hereinafter set forth. As to questions of fact material to this opinion, we have relied upon certificates or comparable documents of public officials and of officers and representatives of the Company.

     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, assumptions and limitations stated herein, we are of the opinion that when the Exchange Securities have been duly executed, authenticated, issued and delivered in accordance with the provisions of the Indenture upon the exchange, the Exchange Securities 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 (i) the effects of bankruptcy, insolvency, fraudulent conveyance, reorganization, moratorium and other similar laws relating to or affecting creditors’ rights generally, (ii) general equitable principles (whether considered in a proceeding in equity or at law) and (iii) an implied covenant of good faith and fair dealing.

     We do not express any opinion herein concerning any law other than the law of the State of New York, the federal law of the United States and the Delaware General Corporation Law

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(including the statutory provisions, all applicable provisions of the Delaware Constitution and reported judicial decisions interpreting the foregoing).

     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 LLP  

  SIMPSON THACHER & BARTLETT LLP