**Exhibit 31.1****CERTIFICATION**     I, Hollings C. Renton, Chairman of the Board, President and Chief Executive Officer of OnyxPharmaceuticals, Inc., certify that:**1.**I have reviewed this Annual Report on Form 10-K of Onyx Pharmaceuticals, Inc. (the“registrant”);**2.**Based on my knowledge, this report does not contain any untrue statement of a material fact oromit to state a material fact necessary to make the statements made, in light of the circumstancesunder which such statements were made, not misleading with respect to the period covered by thisreport;**3.**Based on my knowledge, the financial statements, and other financial information included inthis report, fairly present in all material respects the financial condition, results of operationsand cash flows of the registrant as of, and for, the periods presented in this report;**4.**The registrant’s other certifying officer(s) and I are responsible for establishing andmaintaining disclosure controls and procedures (as defined in Exchange Act Rules 13a-15(e) and15d-15(e)) and internal control over financial reporting (as defined in Exchange Act Rules13a-15(f) and 15d-15(f)) for the registrant and have: a) Designed such disclosure controls and procedures, or caused suchdisclosure controls and procedures to be designed under oursupervision, to ensure that material information relating to theregistrant, including its consolidated subsidiaries, is made known tous by others within those entities, particularly during the period inwhich this report is being prepared;  b) Designed such internal control over financial reporting, or causedsuch internal control over financial reporting to be designed underour supervision, to provide reasonable assurance regarding thereliability of financial reporting and the preparation of financialstatements for external purposes in accordance with generally acceptedaccounting principles;  c) Evaluated the effectiveness of the registrant’s disclosure controlsand procedures and presented in this report our conclusions about theeffectiveness of the disclosure controls and procedures, as of the endof the period covered by this report based on such evaluation; and  d) Disclosed in this report any change in the registrant’s internalcontrol over financial reporting that occurred during the registrant’smost recent fiscal quarter (the registrant’s fourth fiscal quarter inthe case of an annual report) that has materially affected, or isreasonably likely to materially affect, the registrant’s internalcontrol over financial reporting; and**5.**The registrant’s other certifying officer(s) and I have disclosed, based on our most recentevaluation of internal control over financial reporting, to the registrant’s auditors and the auditcommittee of the registrant’s board of directors (or persons performing the equivalent functions): a) All significant deficiencies and material weaknesses in the design oroperation of internal control over financial reporting which arereasonably likely to adversely affect the registrant’s ability torecord, process, summarize and report financial information; and  b) Any fraud, whether or not material, that involves management or otheremployees who have a significant role in the registrant’s internalcontrol over financial reporting.Dated: March 16, 2006         /s/ Hollings C. Renton