In normal men, acute oral administration of 300 mg cimetidine or intravenous injection of 50 mg of the drug had no effect on prolactin release. In contrast, intravenous injection of 150 or 300 mg led to substantial increments in serum prolactin. Peptic ulcer patients were randomly assigned to treatment with either cimetidine or antacid. Serial blood sampling until ulcer healing showed no significant changes in serum prolactin, testosterone, free testosterone, estradiol, LH, or FSH in either group. It is likely that the impotence and breast changes occasionally seen during cimetidine therapy are due to peripheral antagonism of androgen action rather than to alterations in circulating hormone levels.