<PAGE>

                                                                    EXHIBIT 31.1

                          RULE 13A-14(A) CERTIFICATION

                             SIMULATIONS PLUS, INC.
                            a California corporation

                    CERTIFICATION OF CHIEF EXECUTIVE OFFICER

         I, Walter S. Woltosz, Chief Executive Officer of Simulations Plus,
Inc., a California corporation (the "Company"), do hereby certify, in accordance
with Rules 13a-14 and 15d-14, as created pursuant to Section 302(a) of the
Sarbanes-Oxley Act of 2002, with respect to the Quarterly Report on Form 10-QSB
of the Company for the fiscal quarter ended February 29, 2004, as filed with the
Securities and Exchange Commission herewith under Section 15(d) of the
Securities Exchange Act of 1934, as amended (the "Exchange Act"), that:

         (1)      I have reviewed this Quarterly Report on Form 10-QSB of the
                  Company for the fiscal quarter ended February 29, 2004 (the
                  "Quarterly Report");

         (2)      Based on my knowledge, this Quarterly Report does not contain
                  any untrue statement of a material fact or omit to state a
                  material fact necessary to make the statements made, in light
                  of the circumstances under which such statements were made,
                  not misleading with respect to the period covered by this
                  Quarterly Report;

         (3)      Based on my knowledge, the financial statements, and other
                  financial information included in this Quarterly Report,
                  fairly present in all material respects the financial
                  condition, results of operations and cash flows of the Company
                  as of, and for, the periods presented in this Quarterly
                  Report;

         (4)      The Company's other certifying officers and I are responsible
                  for establishing and maintaining disclosure controls and
                  procedures (as defined in Exchange Act Rules 13a-15(e) and
                  15d-15(e)) and internal control over financial reporting (as
                  defined in Exchange Act Rules 13a-15(f) and 15(d)-15(f)) for
                  the Company and have:

                  (a)      designed such disclosure controls and procedures, or
                           caused such disclosure controls and procedures to be
                           designed under our supervision, to ensure that
                           material information relating to the Company,
                           including its consolidated subsidiaries, is made
                           known to us by others within those entities,
                           particularly during the period in which this
                           Quarterly Report is being prepared;

                  (b)      designed such internal control over financial
                           reporting, or caused such internal control over
                           financial reporting to be designed under our
                           supervision, to provide reasonable assurance
                           regarding the reliability of financial reporting and
                           the preparation of financial statements for external
                           purposes in accordance with generally accepted
                           accounting principles;

                  (c)      evaluated the effectiveness of the Company's
                           disclosure controls and procedures and presented in
                           this Quarterly Report our conclusions about the
                           effectiveness of the disclosure controls and
                           procedures, as of the end of the period covered by
                           this Quarterly Report based on such evaluation; and

                  (d)      disclosed in this Quarterly Report any change in the
                           Company's internal control over financial reporting
                           that occurred during the Company's most recent fiscal
                           quarter (the Company's fourth fiscal quarter in the
                           case of an annual report) that has materially
                           affected, or is reasonable likely to materially
                           affect, the Company's internal control over financial
                           reporting; and

         (5)      The Company's other certifying officers and I have disclosed,
                  based on our most recent evaluation, to the Company's auditors
                  and the audit committee of the Company's board of directors
                  (or persons performing the equivalent functions):

                  (a)      all significant deficiencies and material weaknesses
                           in the design or operation of internal control over
                           financial reporting which are reasonably likely to
                           adversely affect the Company's ability to record,
                           process, summarize and report financial information;
                           and

                  (b)      any fraud, whether or not material, that involves
                           management or other employees who have a significant
                           role in the Company's internal control over financial
                           reporting.

Dated: April 12, 2004                           By: /s/ Walter S. Woltosz
                                                    ----------------------------
                                                    Walter S. Woltosz
                                                    Chief Executive Officer