The reason of this study was to evaluate the action of oxidative stress (OS) induced with hydrogen peroxide (H2O2) on the analgesic, antipyretic and anti-inflammatory effects of aspirin in 7-14 day old chicks. H2O2 at 0.5 % v/v in drinking water induced OS in age 7-14 of the chicks when given freshly. The analgesic Median Effective Dose (ED50) and acute Median Lethal Dose (LD50) values of aspirin have been measured here for the first time and show decreased by 28 and 14 % in the stressed (H2O2 group) of chicks, respectively suggesting an increase of aspirin efficacy and lethality at the same time. Furthermore, there is an increase in the analgesic, antipyretic and anti-inflammatory efficacy of aspirin in the H2O2 group of chicks by 228, 275 and 61 %, respectively when compared to the control (H2O group) of chicks. Both plasma AST and ALT of the H2O2 group have a significant increase in concentrations when compared to H2O group of chicks which were both received aspirin at 150 mg/kg, i.m. The data of the present study assume that the analgesic, antipyretic and antiinflammatory effects of aspirin altered by a state of OS induced with H2O2 in the chicks indicating the possible change in the pharmacodynamics and pharmacokinetics of aspirin and therefore, should be recommended here to reduce the dose of aspirin in patients suffering from OS status.