The results of the initial clinical pharmacology studies of famotidine, a new H2 receptor antagonist, are summarized. These studies indicate that: (1) single oral doses of famotidine (5–80 mg) were well tolerated; (2) famotidine effectively suppressed basal, nocturnal, pentagastrin- and meal-stimulated acid secretion; (3) the duration of the antisecretory action of famotidine was dose related: up to 12 h for the 20-mg dosages, and 18-24 h for the 80-mg dose; (4) the elimination half-life was 3.8 h; (5) 5 mg of famotidine gave acid suppression similar to that of 300 mg of cimetidine; (6) a dose response was identified: 2 h after oral dosing, 5 mg famotidine suppressed stimulated acid secretion to 60% of control and was comparable to 300 mg cimetidine (55% suppression), whereas higher doses of famotidine yielded significantly more suppression of acid secretion (10 mg yielding 70% and 20 mg 90%), significantly greater than 300 mg cimetidine; (7) plasma famotidine concentration and urinary recovery were dose related, and (8) famotidine (10–40 mg) had no effect on serum prolactin. Thus, famotidine is a safe and potent H2 blocker of acid secretion with a long duration of action.