The administration of allopurinol significantly increased resistance to repeated hypobaric hypoxia, while hypoxanthine decreased it. The administration of adenine or inosine (25 mg/kg) was without effect. The results show pathogenic significance of xanthine oxidase-dependent production of free oxygen radicals in posthypoxic damage. In other experiments, the administration of 50% ethanol (12 ml/kg) by gastric catheter increased plasma xanthine oxidase activity in both rats and hamsters.