1

                                                                     EXHIBIT 5.1

                                  April 6, 2001

Board of Directors
Zonagen, Inc.
2408 Timberloch Place, Suite B-4
The Woodlands, Texas 77380

Gentlemen:

         We have acted as counsel to Zonagen, Inc. (the "Company") in connection
with the Company's Registration Statement on Form S-8 (the "Registration
Statement") relating to the registration under the Securities Act of 1933, as
amended, of the issuance of up to 650,000 shares (the "Shares") of the Company's
Common Stock, par value $0.001 per share (the "Common Stock"), pursuant to the
Zonagen, Inc. 2000 Nonemployee Directors' Stock Option Plan (the "Director
Plan") and the Zonagen, Inc. 2000 Employee Stock Purchase Plan (together with
the Director Plan, the "Plans").

         In connection herewith, we have examined copies of such statutes,
regulations, corporate records and documents, certificates of public and
corporate officials and other agreements, contracts, documents and instruments
as we have deemed necessary as a basis for the opinion hereafter expressed. In
such examination, we have assumed the genuineness of all signatures, the
authenticity of all documents submitted to us as originals and the conformity
with the original documents of all documents submitted to us as copies. We have
also relied, to the extent we deem such reliance proper, upon information
supplied by officers and employees of the Company with respect to various
factual matters material to our opinion.

         Based upon the foregoing and having due regard for such legal
considerations as we deem relevant, we are of the opinion that the Shares have
been duly authorized, and that such Shares will, when issued in accordance with
the terms of the Plans, be duly authorized, fully paid and validly issued.

         This opinion is limited in all respects to the General Corporation Law
of the State of Delaware and the laws of the United States of America insofar as
such laws are applicable.

         We hereby consent to the use of this opinion as an exhibit to the
Registration Statement. In giving this consent, we do not thereby admit that we
are within the category of persons whose consent is required under Section 7 of
the Securities Act or the rules and regulations of the Securities and Exchange
Commission thereunder.

                                          Very truly yours,



                                          /s/ ANDREWS & KURTH L.L.P.