Document:

FIRST AMENDMENT TO

                        ASSIGNMENT AND SECURITY AGREEMENT

         THIS  FIRST  AMENDMENT  TO  ASSIGNMENT  AND  SECURITY  AGREEMENT  (this
"Agreement")  is made and entered into as of the 1st day of April,  2000, by and
between Prime Medical Operating,  Inc., a Delaware corporation  ("PMOI"),  Prime
RVC,  Inc.,  a Delaware  corporation  ("Prime  RVC"),  as assignee of  Prime/BDR
Acquisition,  L.L.C., a Delaware limited  liability  company ("Prime BDR") (PMOI
and Prime RVC are referred to herein, jointly and severally, as "Secured Party")
and David P. Bates and Jane A. Bates (collectively referred to as the "Debtor").
Unless otherwise  defined herein,  capitalized  terms used herein shall have the
meanings  ascribed to them in that certain  Assignment  and Security  Agreement,
dated effective as of September 1, 1999, among PMOI and Prime RVC as assignee of
Prime BDR (the "Assignment and Security Agreement").

                             Preliminary Statements

         Debtor and Secured Party have executed and delivered that certain Stock
Purchase   Agreement  dated  as  of  September  1,  1999  (the  "Stock  Purchase
Agreement"),  pursuant to which  Secured  Party  purchased  from Debtor  certain
shares of the $0.01 par value  common  stock of Horizon  Vision  Center,  Inc, a
Nevada corporation (the "Existing Center").

         Debtor  pledged  the  collateral  to  secure  certain  obligations  and
liabilities  that Debtor may now or hereafter have to Secured Party,  including,
without limitation,  any indemnity  obligations arising under the Stock Purchase
Agreement.

         The  shareholders  of  the  Existing  Center,  including  Debtor,  have
determined  that it is in their best  interests  that they,  together with Prime
RVC,  organize  and own  Horizon  Vision  Centers,  L.L.C.,  a Delaware  limited
liability  company (the "New  Center")  for the purpose of operating  new vision
centers  that will  provide  refractive  surgery  and  related  services  in the
Northern and Central California area.

         In connection  with the formation of the New Center,  Debtor has agreed
to  pledge  all of its  ownership  interest  in the  New  Center  as  additional
collateral  pursuant to the terms and  conditions of the Assignment and Security
Agreement.

                             Statement of Agreement

         NOW,  THEREFORE,  in consideration of the premises and mutual covenants
contained  herein and for other good,  valuable and binding  consideration,  the
receipt and  sufficiency of which are hereby  acknowledged,  the parties hereto,
intending to be legally bound hereby, agree as follows:

<PAGE>

         Section 1.        Amendments, Additions and Deletions to Agreement.
                           ------------------------------------------------

     a. The parties to this  Agreement  hereby agree to amend Section  1.1(a) of
the Assignment and Security Agreement to read in its entirety as follows:

                  (a) Shares. From and after the date of this Agreement, (i) all
         shares of the common stock of Horizon  Vision  Centers,  Inc., a Nevada
         corporation,  and all membership  interests of Horizon Vision  Centers,
         L.L.C., a Delaware limited liability company,  owned or acquired in any
         manner by Debtor (collectively,  the "Shares"),  (ii) any replacements,
         substitutions,  or  exchanges  of  the  certificates  representing  the
         Shares, and (iii) any and all options,  rescission rights, registration
         rights,   conversion  rights,   subscription  rights,   contractual  or
         quasi-contractual  rights,  warrants,   redemption  rights,  redemption
         proceeds,  calls,  preemptive  rights and all other rights and benefits
         pertaining to the Shares;

         Section  2.  Effect  on  Existing   Agreements.   This   Agreement   is
incorporated into the Assignment and Security Agreement by reference. Other than
as provided in this  Agreement,  the Assignment  and Security  Agreement has not
been  modified or amended and is in full force and effect.  Equity Holder hereby
affirms that it remains a party to the  Assignment  and Security  Agreement  (as
amended by this Agreement) after the execution of this Agreement. This Agreement
may be executed in a number of identical  counterparts  which,  taken  together,
shall constitute collectively one and the same agreement.

                            [Signature page follows]

<PAGE>

S-1

043838.0009  AUSTIN 189916 v1

                                SIGNATURE PAGE TO

                                 FIRST AMENDMENT

                                       TO

                        ASSIGNMENT AND SECURITY AGREEMENT

         IN  WITNESS  WHEREOF,  the  parties  hereto  have  duly  executed  this
Amendment as of the day and year first above written.

DEBTOR:

                                                David P. Bates, III

                                                Jane A. Bates

SECURED PARTY:                                  PRIME MEDICAL OPERATING, INC.

                                            By:_________________________________
                                               Teena Belcik, Treasurer

                                                PRIME/BDR ACQUISITION, L.L.C.

                                            By:_________________________________
                                               Teena Belcik, Treasurer

                                                PRIME RVC, INC.

                                            By:_________________________________
                                               Teena Belcik, TreasurerCONSENT AND AGREEMENT

         This Consent and Agreement (this "Agreement") is entered into as of the
1st day of April,  2000, by the  undersigned  shareholder  ("Equity  Holder") of
Horizon Vision Centers,  Inc., a Nevada corporation (the "Existing Center"), for
the benefit of the Existing Center,  Horizon Vision Centers,  L.L.C., a Delaware
limited  liability  company  (the "New  Center"),  Prime RVC,  Inc.,  a Delaware
corporation ("Prime RVC") and the parent companies and affiliates of each of the
Existing  Center,  the New  Center  and Prime  RVC.  As used in this  Agreement,
"Equity  Holder" shall, in the case of a shareholder of the Existing Center that
is  not  an  individual  physician,  refer  also  to  the  individual  physician
affiliated  with  such  shareholder  of the  Existing  Center,  both of whom are
parties to this Agreement.

                             Preliminary Statements

         Equity Holder desires to  acknowledge  and consent to the assignment by
Prime BDR to Prime RVC of any and all rights that Prime BDR may have pursuant to
all contracts or agreements  executed by Equity Holder in connection with Equity
Holder's  prior sale of stock of the  Existing  Center to Prime BDR,  including,
without  limitation,  the  related  stock  purchase  agreement,  assignment  and
security agreement,  and financing statement, as applicable  (collectively,  the
"Acquisition Documents").

         Equity Holder  desires to  acknowledge  and consent,  in its respective
capacities as a shareholder,  member and (as applicable) officer of the Existing
Center and the New Center,  to the assignment by the Existing  Center to the New
Center of  certain  assets  and the  assumption  by the New  Center  of  certain
obligations related to those assets.

         The shareholders of the Existing Center,  including Equity Holder, have
determined  that it is in their best  interests  that they,  together with Prime
RVC,  organize  and own the New Center for the purpose of  operating  new vision
centers  that will  provide  refractive  surgery  and  related  services  in the
Northern and Central California area.

         In order to operate the New Center,  the parties to this Agreement deem
it necessary to amend that certain Stock Purchase Agreement,  dated effective as
of September 1, 1999,  among Equity Holder,  Prime/BDR  Acquisition,  L.L.C.,  a
Delaware  limited  liability  company ("Prime BDR") and the Existing Center (the
"Stock Purchase Agreement"),  to allow Equity Holder and other physicians to use
the Refractive Surgery Services of the New Center.

                             Statement of Agreement

         NOW,  THEREFORE,  in consideration of the premises and mutual covenants
contained  herein and for other good,  valuable and binding  consideration,  the
receipt and  sufficiency of which are hereby  acknowledged,  the parties hereto,
intending to be legally bound hereby, agree as follows:

         Section 1.        [This Section intentionally left blank]

               Section 2. Amendments to Stock Purchase Agreement. The parties to
          this  Agreement  hereby  agree to amend  Section  8.2(a)  of the Stock
          Purchase Agreement to read in its entirety as follows:

               (a) Except through the Company, engage in or provide any services
          that are provided by the  Company,  directly or  indirectly,  anywhere
          within a two  hundred  (200) mile  radius of any center or facility at
          any time operated by the Company or any of the  Company's  affiliates,
          including,  without  limitation,  any  services  related  to,  (i) the
          operating of laser refractive surgical centers,  (ii) the manufacture,
          maintenance,  refurbishing,  repair, sale, or leasing of any equipment
          related to or necessary for the operating of laser refractive surgical
          centers,  or (iii)  providing  any  management  services,  training or
          consulting services related to any of the activities  described in (i)
          or (ii);

         Section 3. Consent to Assignment. Notwithstanding any provisions of the
Acquisition  Documents to the contrary,  Equity Holder hereby  consents,  in its
respective  capacities as a shareholder,  member and (as applicable)  officer of
the Existing Center and the New Center, to:

                  (a)  the   assignment  by  Prime  BDR  to  Prime  RVC  of  the
Acquisition Documents,  and any and all rights that Prime BDR might have arising
under or related to the Acquisition Documents;

               (b)   the   New   Center's   execution   and   delivery   of  the
          Non-Competition Agreement attached as Exhibit A to this Agreement;

                  (c) the Existing Center's and New Center's execution, delivery
and performance of the Assignment and Assumption Agreement attached as Exhibit B
to this Agreement.

         Equity Holder hereby  ratifies any action taken by the Existing  Center
or the New Center prior to the date of this  Agreement to the extent such action
would have been properly  authorized  pursuant to this Agreement.  Equity Holder
hereby agrees to take such other  actions,  including the execution and delivery
of such other  documents  or  instruments,  as may  reasonably  be  requested by
another  party to this  Agreement  in order to fully carry out the intent of the
provisions of this Agreement.

<PAGE>

         Section 4.  Effect on Existing  Agreements.  This  Agreement  is hereby
incorporated  into the Exclusive Use Agreement and the Stock Purchase  Agreement
by reference.  Other than as provided in this  Agreement,  neither the Exclusive
Use Agreement nor the Stock Purchase  Agreement has been modified or amended and
each is in full force and effect.  Equity  Holder  hereby  affirms that it is or
remains a party to the Exclusive Use Agreement and the Stock Purchase  Agreement
(each as amended by this Agreement) after the execution of this Agreement.  This
Agreement  may be executed in a number of identical  counterparts  which,  taken
together, shall constitute collectively one and the same agreement.

                           [Signature page to follow]

<PAGE>

S-1

043838.0009  AUSTIN 189946 v1

                                SIGNATURE PAGE TO

                              CONSENT AND AGREEMENT

         IN  WITNESS  WHEREOF,  the  parties  hereto  have  duly  executed  this
Agreement as of the day and year first above written.

EQUITY HOLDER:

                                   Signature:

                                                   David P. Bates III

                                   Signature:

                                                   Jane A. Bates

EXISTING CENTER:                                  Horizon Vision Centers, Inc.

                                                  Teena Belcik, Treasurer

NEW CENTER:                                       Horizon Vision Centers, L.L.C.

                                                  Teena Belcik, Treasurer

PRIME BDR:                                        Prime/BDR Acquisition, L.L.C.

                                                  Teena Belcik, Treasurer

PRIME RVC:                                        Prime RVC, Inc.

                                                  Teena Belcik, Treasurer

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