The sexual dimorphism of gastroduodenal ulceration is suggested on the basis of clinical and experimental observations. This difference probably relates to the actions of endogenous sexual steroids. In the present study, the role of testosterone was evaluated in the generation of gastroduodenal mucosal injury provoked by cysteamine (400 mg/kg, s.c.). We found that macroscopic mucosal damage and microvascular (125)I-human serum albumin leakage (2 microCi/kg, i.v.) developed in the stomach and duodenum of male rats 24 h after the administration of cysteamine. This mucosal injury was prevented by orchidectomy and by the pretreatment with the antiandrogen, cyproterone acetate (12 mg/kg per day for 8 consecutive days). It was also shown that pretreatment with testosterone (4-20 mg/kg per week) dose-dependently aggravated cysteamine-induced gastroduodenal mucosal injury. Our results thus suggest an aggressive role of testosterone in the generation of cysteamine-induced gastroduodenal ulceration.