<PAGE>
 
                                                                    EXHIBIT 3(a)

         FILED
   IN THE OFFICE OF THE
SECRETARY OF STATE OF THE
    STATE OF NEVADA
                           
       MAY 22 1998                
NO. C15780-93              
    ---------              
    /s/DEAN HELLER
DEAN HELLER, SECRETARY OF STATE

                           CERTIFICATE OF AMENDMENT
                         
                                      OF
                         
                           ARTICLES OF INCORPORATION

                                      OF

                                  CLINICOR, INC.


     The undersigned, being the President and Assistant Secretary of CLINICOR,
INC., do certify and set forth:

     1.  The name of the Corporation is CLINICOR, INC.

     2.  Section 1 and the first paragraph of Section 2 of Article VI of the
Articles of Incorporation of the Corporation shall be amended to read in their
entirety as follows:

          Section 1.  Authorized Shares.  The aggregate number of shares which
                      -----------------                                       
     the Corporation shall have authority to issue is 75,155,181, of which (a)
     75,000,000 shares shall be designated "Common Stock", par value $.001 per
     share, (b) 5,181 shares shall be designated "Class A Convertible Preferred
     Stock", without par value, (c) 50,000 shares shall be designated "Class B
     Convertible Preferred Stock", without par value, and (d) 100,000 shares
     shall be designated "Class C Serial Preferred Stock", without par value.
     The "Class A Convertible Preferred Stock" is sometimes referred to as the
     "Class A Preferred Stock", the "Class B Convertible Preferred Stock" is
     sometimes referred to as the "Class B Preferred Stock", and both of such
     classes of stock are sometimes referred to collectively as the "Convertible
     Preferred Stock."  The "Class C Serial Preferred Stock" is sometimes
     referred to as the "Class C Preferred Stock."  The "Convertible Preferred
     Stock" and "Class C Preferred Stock" are sometimes referred to collectively
     as the "Preferred Stock".

          Section 2.  Relative Rights.  Subject to the rights of the holders of
                      ---------------                                          
     the Preferred Stock, the Common Stock shall be entitled to dividends out of
     funds legally available therefor, when, as and if declared and paid to the
     holders of Common Stock, and upon liquidation, dissolution or winding up of
     the Corporation, to share ratably in the assets of the Corporation
     available for distribution to the holders of Common Stock.  Except as
     otherwise provided herein or by law, the holders of the Common Stock shall
     have full voting rights and powers and each share of Common Stock shall be
     entitled to one vote.  The following is a statement of the initial
     designations, preferences, limitations and relative rights in respect of
     the shares of each class of Convertible Preferred Stock of the Corporation
     and the rights of the Board of Directors to designate the voting powers,
     designations, preferences, limitations, restrictions and relative or other
     rights of each series of Class C Preferred
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     Stock and to alter the voting powers, designations, preferences,
     limitations, restrictions and relative or other rights of the Convertible
     Preferred Stock.  Unless otherwise indicated, references to "Sections"
     contained herein shall refer to subdivisions of this Section 2 of Article
     VI.

     3.   The last sentence of Section A(3)(a) and Section B(2)(a) of Section 2
of Article VI of the Articles of Incorporation shall be amended to read in its
entirety as follows:

     In any event, if the assets of the Corporation available for distribution
     to the holders of Convertible Preferred Stock shall exceed the distribution
     required to be made to the holders of Class A Preferred Stock and the Class
     B Preferred Stock as herein described, such excess assets shall be
     distributed pro-rata among the holders of Common Stock, subject to the
     rights of the holders of any Class C Preferred Stock.

     4.   There shall be added a new Subsection C of Section 2 of Article VI of
the Articles of Incorporation, which shall read in its entirety as follows:

          C.  Class C Preferred Stock.
              ----------------------- 

              The Board of Directors of the Corporation is authorized at any
     time, and from time to time, to provide for the issuance of shares of Class
     C Preferred Stock in one or more series.  For each series, the Board of
     Directors shall determine, by resolution or resolutions adopted prior to
     the issuance of any shares thereof, the voting powers, designations,
     preferences, limitations, restrictions and relative or other rights
     thereof, including but not limited to the following relative rights and
     preferences, as to which there may be variations among different series:

               1.  The rate and manner of payment of dividends, if any;

               2.  Whether shares may be redeemed and, if so, the redemption
     price and the terms and conditions of redemption;

               3.  The amount payable for shares in the event of liquidation,
     dissolution or other winding up of the Corporation;

               4.  Sinking fund provisions, if any, for the redemption or
     purchase of shares;

               5.  The terms and conditions, if any, on which shares may be
     converted or exchanged;

               6.  Voting rights, if any; and

                                       2
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               7. Any other rights or preferences of such shares, to the full
     extent now or hereafter permitted by the laws of the State of Nevada.

               The Board of Directors shall have the authority to determine the
     number of shares that will comprise each series.

               Each series of Class C Preferred Stock may be subordinate or
     senior to or pari passu with the Class A Preferred Stock and the Class B
     Preferred Stock, as the Board of Directors may determine.  The resolution
     or resolutions of the Board of Directors setting forth the voting powers,
     designations, preferences, limitations, restrictions and relative or other
     rights of any series of Class C Preferred Stock may also modify the voting
     powers, designations, preferences, limitations, restrictions and relative
     or other rights of the Convertible Preferred Stock.

               Prior to the issuance of any shares of a series, but after
     adoption by the Board of Directors of the resolution establishing such
     series, the appropriate officers of the Corporation shall file such
     documents with the State of Nevada as may be required by law.

               Notwithstanding anything herein to the contrary, no series of
     Class C Preferred Stock may be established unless the resolutions setting
     forth the voting powers, designations, preferences, limitations,
     restrictions and relative or other rights of such series of Class C
     Preferred Stock (including any modification of the voting powers,
     designations, preferences, limitations, restrictions and relative or other
     rights of the Convertible Preferred Stock) are approved, in the case of
     each and every series of Class C Preferred Stock, by the affirmative vote
     or consent of the holders of at least two-thirds of the shares of Class A
     Preferred Stock at the time outstanding, voting separately as a class, and
     by the affirmative vote or consent of the holders of at least a majority of
     the shares of Class B Preferred Stock then outstanding, voting separately
     as a class.

     5.   The amendment effected herein was authorized by resolutions adopted at
a meeting on March 30, 1998 of the Board of Directors pursuant to Section 78.315
of the Nevada Revised Statutes, declaring the advisability thereof and calling a
meeting of the shareholders for the purpose of considering and voting on the
proposed amendment.

     6.   The number of shares of the Corporation outstanding and entitled to
vote on the amendment to the articles of incorporation at the time of adoption
was 4,136,400 shares of Common Stock with one (1) vote per share, 3,930 shares
of Class A Convertible Preferred Stock with six hundred sixty-six and two-thirds
(666-2/3) votes per share, and 50,000 shares of Class B Convertible Preferred
Stock with thirty-three and one-third (33-1/3) votes per share.  The foregoing
amendment was adopted by the shareholders of the corporation on May 19, 1998.

                                       3
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     7.   The number of shares of each class voted for and against such
amendment, respectively was:
 
                                           NUMBER OF SHARES VOTED
                                           ----------------------
     CLASS                                     FOR     AGAINST
     -----                                     ---     -------
 
     Common Stock                           2,357,139   87,120
 
     Class A Convertible Preferred Stock        3,930     -0-
 
     Class B Convertible Preferred Stock       50,000     -0-
 
     IN WITNESS WHEREOF, we have executed this Certificate on this 20th day of
                                                                   ----       
May, 1998.

                                       CLINICOR, INC.


 
                                       By:    /s/ Robert S. Sammis
                                          --------------------------------------
                                          Robert S. Sammis, President


    /s/ Lori Melton
-----------------------------------
Lori Melton, Assistant Secretary



STATE OF TEXAS      (S)
                    (S)
COUNTY OF TRAVIS    (S)

     This instrument was acknowledged before me on the 20th day of May, 1998, by
                                                       ----                     
Robert S. Sammis as President of Clinicor, Inc.


                                       /s/ Marilyn S. Deavours
                                       ----------------------------------------
                                       NOTARY PUBLIC, STATE OF TEXAS

--------------------------
      MARILYN S. DEAVOURS
SEAL     NOTARY PUBLIC
         STATE OF TEXAS
     COMM. EXP. 05-22-2001
--------------------------

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