**Exhibit 31.1**

 

**CERTIFICATION**

**OF CHIEF EXECUTIVE OFFICER**

 

I, Kevin F. McLaughlin, certifythat:

 

1.             I have reviewed this quarterly report on Form 10-Q ofPRAECIS PHARMACEUTICALS INCORPORATED;

 

2.             Based on my knowledge, this report does not contain anyuntrue statement of a material fact or omit to state a material fact necessaryto make the statements made, in light of the circumstances under which suchstatements were made, not misleading with respect to the period covered by thisreport;

 

3.             Based on my knowledge, the financial statements, andother financial information included in this report, fairly present in allmaterial respects the financial condition, results of operations and cash flowsof the registrant as of, and for, the periods presented in this report;

 

4.             The registrant’s other certifying officer and I areresponsible for establishing and maintaining disclosure controls and procedures(as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal controlover financial reporting (as defined in Exchange Act Rules 13a-15(f) and15d-15(f)) for the registrant and have:

 

(a) Designedsuch disclosure controls and procedures, or caused such disclosure controls andprocedures to be designed under our supervision, to ensure that material informationrelating to the registrant, including its consolidated subsidiaries, is madeknown to us by others within those entities, particularly during the period inwhich this report is being prepared;

 

(b) Designedsuch internal control over financial reporting, or caused such internal controlover financial reporting to be designed under our supervision, to providereasonable assurance regarding the reliability of financial reporting and thepreparation of financial statements for external purposes in accordance withgenerally accepted accounting principles;

 

(c) Evaluatedthe effectiveness of the registrant’s disclosure controls and procedures andpresented in this report our conclusions about the effectiveness of thedisclosure controls and procedures, as of the end of the period covered by thisreport based on such evaluation; and

 

(d) Disclosedin this report any change in the registrant’s internal control over financialreporting that occurred during the registrant’s most recent fiscal quarter (theregistrant’s fourth fiscal quarter in the case of an annual report) that hasmaterially affected, or is reasonably likely to materially affect, theregistrant’s internal control over financial reporting; and

 

5.             The registrant’s other certifying officer and I havedisclosed, based on our most recent evaluation of internal control overfinancial reporting, to the registrant’s auditors and the audit committee ofthe registrant’s board of directors (or persons performing the equivalentfunctions):

 

(a) All significantdeficiencies and material weaknesses in the design or operation of internalcontrol over financial reporting which are reasonably likely to adverselyaffect the registrant’s ability to record, process, summarize and reportfinancial information; and

 

(b) Any fraud,whether or not material, that involves management or other employees who have asignificant role in the registrant’s internal control over financial reporting.

 

Date:       August 5, 2005

 
| /s/ Kevin F.  McLaughlin |   |
|--------------------------|--------|
| Kevin F.  McLaughlin     |
| Chief Executive  Officer |

 

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