The effect of thyroid hormones on the hepatic xanthine oxidase activity was studied in rats after the intraperitoneal injections of comthyroid (triiodotyronine:thyroxine=1:4) at 0.3 mg/kg for 3 consecutive days. The aim of this study was to understand the precise mechanism of hyperthyroidism induced by oxidative stress. The concentration of lipid peroxides determined indirectly by the measurement of thiobarbituric acid reactants was increased in comthyroid treated rats. The hepatic glutathione content was decreased in comthyroid injected rat compared to the euthyroid state. It was also observed that the increment of xanthine oxidase activity has a profound role in oxygen radicals generation system in comthyroid treated rat. These findings suggest that the enhanced xanthine oxidase activity and depleting glutathione content in comthyroid treated rats result in pathophysiological oxidative stress including an increment of hepatic lipid peroxidation.