The effect of a single bedtime dose of famotidine 40 mg on gonadal function was studied in 8 male duodenal ulcer patients. The drug was orally administered for 4 weeks. Our results show that this new H2 blocker influences basal and stimulated serum levels of neither testosterone nor gonadotrophins (LH, FSH). Besides, no significant variations were observed before and after famotidine treatment in seminal fluid characteristics evaluated in 5 out of 8 cases. It can be concluded that famotidine appears to leave gonadal function unaffected in man.