In 20 patients with duodenal ulcer we measured serum prolactin levels following acute and long-term cimetidine administration. In addition, in 20 healthy volunteers we studied the effect of pre-treatment with bromocriptine, meterogline, nomifensine and cryroheptadine on cimetidine-induced prolactin release. Intravenous cimetidine stimulated prolactin secretion in patients and in normal subjects. In the latter, bromocroptine and metergoline pre-administration blunted the release of prolactin in response to iv cimetidine whereas nomifensine and cyproheptadine were ineffective. Long-term treatment with cimetidine (1.2 g daily for 3 months) had no effect on prolactin secretion in the 20 patients studied. No incidence of gynaecomastia, galactorrhoea or disorders of the menstual cycle was observed.