Exhibit 4.4
                                                                     -----------


                       AMENDMENT NO. 1 TO RIGHTS AGREEMENT

            AMENDMENT NO. 1 (this "Amendment"), dated as of May 4, 2001, to the
Rights Agreement (the "Rights Agreement"), dated as of July 17, 1997, by and
between Valero Energy Corporation (the "Company") and Computershare Investor
Services, LLC, as successor rights agent to Harris Trust and Savings Bank (the
"Rights Agent").

            WHEREAS, the Company and the Rights Agent have heretofore executed
and entered into the Rights Agreement; and

            WHEREAS, pursuant to Section 27 of the Rights Agreement, the Company
may from time to time supplement or amend the Rights Agreement in accordance
with the provisions of Section 27 thereof; and

            WHEREAS, the Company intends to enter into an Agreement and Plan of
Merger (as it may be amended or supplemented from time to time, the "Merger
Agreement"), to be dated as of May 6, 2001, by and between the Company and UDS
(all capitalized terms used in this Amendment and not otherwise defined herein
shall have the meaning ascribed thereto in the Merger Agreement); and

            WHEREAS, the Board of Directors has determined that the Agreement
and the transactions contemplated thereby are advisable, fair to and in the best
interests of the Company and its stockholders; and

            WHEREAS, the Board of Directors has determined that it is desirable
to amend the Rights Agreement to exempt the Merger Agreement from the
application of the Rights Agreement.

            NOW, THEREFORE, the Company hereby amends the Rights Agreement as
follows:

            1. The definition of "Acquiring Person" contained in Section 1(a) of
the Rights Agreement is hereby modified and amended by adding the following
sentence at the end thereof:

            "Notwithstanding the foregoing, UDS shall not be deemed to be an
            Acquiring Person by virtue of the execution and delivery of the
            Agreement and Plan of Merger (the "Merger Agreement") to be entered
            into as of May 6, 2001, by and between the Company and UDS, or as a
            result of the consummation of the transactions contemplated by the
            Merger Agreement."

            2. The definition of "Shares Acquisition Date" contained in Section
1(l) of the Rights Agreement is hereby modified and amended by adding the
following sentence at the end thereof:

            "Notwithstanding the foregoing, neither the execution and delivery
            of the Merger Agreement, nor consummation of the transactions
            contemplated thereby, shall cause a Shares Acquisition Date."






            3. Section 3(a) of the Rights Agreement is hereby modified and
amended by adding the following sentence immediately following the first
sentence thereof:

            "Notwithstanding the foregoing, neither the execution and delivery
            of the Merger Agreement, nor consummation of the transactions
            contemplated thereby, shall cause a Distribution Date."

            4.  Section 15 of the Rights Agreement is hereby modified and
amended by adding the following sentence at the end thereof:

            "Nothing in this Agreement shall be construed to give any holder of
            Rights or any other Person any legal or equitable rights, remedy or
            claim under this Agreement in connection with any transactions
            contemplated by the Merger Agreement."

            5. This Amendment shall be deemed to be a contract made under the
laws of the State of Delaware and for all purposes shall be governed by and
construed in accordance with the laws of such State applicable to contracts to
be made and performed entirely within such State.

            6. This Amendment may be executed in any number of counterparts and
each of such counterparts shall for all purposes be deemed to be an original,
and all such counterparts shall together constitute but one and the same
instrument.

            7. In all respects not inconsistent with the terms and provisions of
this Amendment, the Rights Agreement is hereby ratified, adopted, approved and
confirmed. In executing and delivering this Amendment, the Rights Agent shall be
entitled to all the privileges and immunities afforded to the Rights Agent under
the terms and conditions of the Rights Agreement.

            8. If any term, provision, covenant or restriction of this Amendment
is held by a court of competent jurisdiction or other authority to be invalid,
void or unenforceable, the remainder of the terms, provisions, covenants and
restrictions of this Amendment, and of the Rights Agreement, shall remain in
full force and effect and shall in no way be affected, impaired or invalidated.

            9. Except as otherwise expressly provided herein, or unless the
context otherwise requires, all terms used herein have the meanings assigned to
them in the Rights Agreement.

            10. The Rights Agent and the Company hereby waive any notice
requirement under the Rights Agreement pertaining to the matters covered by this
Amendment.


                               *        *        *





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            IN WITNESS WHEREOF, this Amendment No. 1 has been duly executed by
the Company and the Rights Agent as of the day and year first written above.



                                    VALERO ENERGY CORPORATION

                                    By: /s/ Michael S. Ciskowski
                                       -------------------------
                                    Name:   Michael S. Ciskowski
                                    Title:  Senior Vice President





                                    COMPUTERSHARE INVESTOR SERVICES,
                                    LLC (as Rights Agent)

                                    By: /s/ Gerald K. Lane
                                       ---------------------------
                                    Name:   Gerald K. Lane
                                    Title:  Relationship Manager




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