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                                                                     EXHIBIT 5.1




                                August 20, 1997

Board of Directors
Administaff, Inc.
19001 Crescent Springs Drive
Kingwood, Texas 77339-3802

Ladies and Gentlemen:

                 We have acted as counsel to Administaff, 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 882,165 shares (the "Shares") of
the Company's common stock, par value $.01 per share, pursuant to the
Administaff, Inc. 1997 Incentive Plan (the "Plan").

                 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 reserved for issuance and, when issued in accordance
with the terms of the Plan, will be validly issued, fully-paid and
nonassessable.

                 We hereby consent to the use of this opinion as an exhibit to
the Registration Statement.

                                                   Very truly yours,


                                                   /S/ ANDREWS & KURTH L.L.P.

1198/2325/2608