Ulcer formation after pylorus ligation was assessed in control, testosterone treated and castrated male rats after cimetidine treatment. The stomach was studied for incidence of ulcers and its contents analysed for pH, volume, total acidity, free acidity, pepsin and mucin activity. Testosterone and cimetidine when used alone protected from ulceration while when used in combination the degree of protection was decreased. Castration per se ead no effect on ulcer index but potentiated cimetidine induced gastric ulcer protection.