Exhibit
31.1
 
  I, George
Paz, certify that:
  1.
    I
      have reviewed this quarterly report on Form 10-Q of Express Scripts,
      Inc.;
  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
      registrant as of, and for, the periods presented in this quarterly report;
  4.
    The
      registrant'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), for the registrant
      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 registrant, 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 registrant'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 report based on such evaluation;
      and
  d)
    disclosed
      in this quarterly report any change in the registrant's internal control
      over financial reporting that occurred during the registrant's most recent
      fiscal quarter (the registrant's fourth fiscal quarter in the case of an
      annual report) that has materially affected, or is reasonably likely to
      materially affect, the registrant's internal control over financial
      reporting; and
  5.
    The
      registrant's other certifying officers and I have disclosed, based on our
      most recent evaluation of internal control over financial reporting, to
      the registrant's auditors and the audit committee of registrant's board of
      directors (or persons performing the equivalent function):
 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 registrant'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 registrant's internal control
      over financial reporting.
  Date:
April 26, 2005
   /s/
George
Paz
 George
Paz, President and
 Chief
Executive Officer