This study was designed to determine whether oxygen-derived free radicals play a role in the pathogenesis of gastric lesions produced by hypotensive ischemia in the rat. To achieve this goal, allopurinol, an inhibitor of xanthine oxidase (the enzyme responsible for the formation of superoxide radicals); superoxide dismutase, a scavenger of superoxide radicals (O2-); and dimethyl sulfoxide, a scavenger of hydroxyl radicals (OH) were used. In the anesthetized rat, HCl (0.1 N) was instilled into the pylorus-ligated stomach, and the rat was bled to reduce the blood pressure to less than 30 mmHg. The blood pressure was maintained at less than 30 mmHg for 20 min and then the shed blood was retransfused. Twenty minutes after the retransfusion the rat was killed, the stomach was removed, and the area of gastric mucosal lesions was measured. Both allopurinol and superoxide dismutase, but not dimethyl sulfoxide, significantly protected against hemorrhagic shock-induced gastric lesions. These findings suggest that oxygen-derived free radicals, particularly O2-, play an important role in the formation of gastric lesions produced by ischemia plus HCl.