Cimetidine, a non-thiourea-containing H2-receptor antagonist, was studied in seven patients with duodenal ulcer. Oral doses of 100, 200, and 300 mg were tested. Each dose significantly inhibited basal and meal-stimulated secretion. After 300 mg, basal acid secretion was essentially zero for at least five hours. The meal-stimulated three-hour acid output after the 300-mg dose was reduced by 67%. Cimetidine, 300 mg, decreased meal-stimulated acid secretion significantly more than an optimal effective dose of propantheline bromide (P less than 0.05). Inhibition of meal-stimualted gastric acid secretion showed a significant relation to peak blood cimetidine concentration (r is equal to 0.76, P less than 0.01). Cimetidine did not affect meal-stimulated gastrin release. No toxicity was observed after serial doses given during these tests. Cimetidine may be useful in treatment of acid-peptic diseases provided no important toxicity appears on chronic testing.