The effect of cimetidine 1600 mg daily for 3 mo. on prolactin [PRL] and related hormones [in peptic ulcer patients] is reported. Basal PRL levels rose slightly but not significantly. There was no change in basal thyroid [thyroxine, triiodothyronine] and sex hormones [estradiol, androgen] levels nor in the PRL, gonadotropin or thyrotropin responses to releasing hormone [thyroliberin] stimulation. Since i.v. cimetidine induces a transient hyperprolactinemia it appears that cimetidine may facilitate release of PRL but has no effect on its synthesis. Gynecomastia is a rare complication of cimetidine. Drug-induced hyperprolactimemia was suggested as the mechanism, although this is doubtful. Testing PRL responses to exogenous releasing hormones rather than measuring basal levels may explain some of the anomalies reported.