<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 Annual Report on Form 10-KSB of
the Company for the fiscal year ended August 31, 2007, 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 Annual Report on Form 10-KSB of the Company for
            the fiscal year ended August 31, 2007 (the "Annual Report");

      (2)   Based on my knowledge, this Annual 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 Annual Report;

      (3)   Based on my knowledge, the financial statements, and other financial
            information included in this Annual 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 Annual 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 15d-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 Annual 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 Annual Report
                  our conclusions about the effectiveness of the disclosure
                  controls and procedures, as of the end of the period covered
                  by this Annual Report based on such evaluation; and

            d.    disclosed in this Annual 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: November 23, 2007                        By: /s/ Walter S. Woltosz
                                                    ---------------------
                                                    Walter S. Woltosz
                                                    Chief Executive Officer