The role of oxygen derived free radicals in gastric lesions induced by haemorrhagic shock and the protective effect of oxygen radical scavengers, allopurinol and ranitidine, were investigated. Forty five rabbits underwent haemorrhagic shock for 30 minutes and reinfusion of shed blood. They were killed 30 minutes later. The animals were divided in five groups: A (n = 10): Control, B (n = 10): intravenous ranitidine pretreatment, C (n = 10): oral allopurinol, 24 and 2 h before surgery; D (n = 10): intravenous pretreatment with superoxide Dismutase plus catalase, E (n = 5): 60 minute haemorrhagic shock without reinfusion and treatment. Erosions and/or petechiae in all animals in Group A were observed. Three animals in group B and C and 2 in group D (p less than 0.005, p less than 0.001) had gastric lesions. The lesions in the pretreatment groups were significantly smaller than in controls. Oxygen radicals plus HCl play an important role in shock induced gastric lesions. Oxygen radical antagonists show a significant protective role.