The effect of oxidative stress–induced by hydrogen peroxide (H2O2) on the analgesic effect of xylazine and dipyrone in 7-14 days old chicks was studied, compared with the control group that given plane tap water. H2O2, 0.5 % in water, induced oxidative stress in chicks by significantly lowering glutathione, rising malondialdehyde in plasma, whole brain during the day 7th, 10th, 14th of chicks old in comparison with the control group. The analgesic median effective doses (ED50) of xylazine and dipyrone in the control group were determined to be 0.79 and 65.3 mg/kg, intramuscularly (i.m.), respectively whereas H2O2 treated groups decreased these values to be 0.31 and 37.2 mg/kg, i.m. by 61 and 43%, respectively. Intramuscular injection of xylazine and dipyrone at 0.5, 70 mg/kg respectively causes analgesia from electro-stimulation induced pain in 50, 66.67% respectively in control groups whereas H2O2 treated chicks increases the analgesic efficacy to be 83.33 and 83.33% respectively. Xylazine and dipyrone injection at 1 and 100 mg/kg, i.m. 15 minutes before formaldehyde injection in right planter foot of stressed chicks causes analgesia from pain induced by formaldehyde through significant increases in onset of lifting of formaldehyde injected foot, significantly decreases its lifting numbers, decreases the time elapsed of lifting of formaldehyde injected foot in comparison with the stressed control group that injected with saline in right planter foot. The data of this study indicate that H2O2-induced oxidative stress potentiate the analgesic efficacy of the central and peripheral analgesics of xylazine and dipyrone in chicks.