Meal-stimulated acid secretion, measured by in vivo intragastric titration, was progressively inhibited by increasing oral doses of cimetidine (25 to 400 mg). Four hundred milligrams suppressed acid secretion by 73% for the first 3 hr after the meal, whereas it inhibited acid secretion by 94% during the 30-min period of maximal inhibition. The dose of cimetidine required to suppress acid secretion by 50% during the 30-min period of maximal inhibition was 25 mg. The duration of action of a 300-mg dose was at least 7 hr. Cimetidine was equally effective in inhibiting meal-stimulated acid secretion at two physiological intragastric pH levels (5.0 and 2.5). Cimetidine had no effect on serum gastrin concentration when intragastric pH was maintained at 5.0, but when pH was allowed to seek its own level, serum gastrin concentration was higher after cimetidine than after placebo. Cimetidine had no effect on gastric emptying. No side effects were noted in any patients.