The effect of cimetidine maintenance treatment on gonadal function has been assessed by seminal analysis plus prolactin and gonadotropin blood level concentrations. Nine patients, all with duodenal ulcer, 5 of whom had received 400 mg cimetidine at night for 18, 3 for 24 and 1 for 36 months, were studied. All patients were presumably fertile, having fathered a child born not more than 3 years before the start of cimetidine treatment. In all patients two seminal analyses were performed and blood specimens were obtained for radioimmunoassay estimation of FSH, LH and prolactin concentrations both in basal conditions and 20', 30' and 60' after the i.v. injection of Gn-RH (100 micrograms) and TRH (200 micrograms). All but one patient, in whom mild oligoasthenozoospermia was detected, had normal sperm counts and motility; moreover hormonal levels both in basal conditions and after releasing factor stimulation were found to be similar to those observed in a control group. Cimetidine maintenance treatment has no effect on sexual activity, the quality of seminal fluid or the pituitary secretion of gonadotropins or prolactin.