Ischemia and reperfusion are of the greatest importance in the pathology of many diseases. We investigated the roles of oxygen-derived free radicals and lipid peroxidation in gastric mucosal injury, such as spotty and linear hemorrhagic erosions, induced in rats by ischemia-reperfusion. The gastric mucosal injury and the increase in thiobarbituric acid (TBA) reactants in the gastric mucosa induced by ischemia-reperfusion were significantly inhibited by treatment with SOD and catalase. These results suggest that oxygen-derived free radicals and lipid peroxidation play important roles in the pathogenesis of acute gastric mucosal lesions induced by ischemia-reperfusion.