Oral administration of YM-11170 (5-20 mg) inhibited both basal and tetragastrin-induced gastric secretion of acid and pepsin in healthy volunteers. YM-11170 was at least 20 times more potent than cimetidine in inhibiting stimulated acid secretion. The area under the plasma concentration of YM-11170 vs time curve correlated positively to both dose and percent inhibition of acid output in response to tetragastrin. YM-11170 significantly inhibited basal and stimulated acid secretion even 10 h after a 20-mg dose. A plasma level of YM-11170 required for 50% inhibition of stimulated acid secretion was found to be 13 ng/ml. These results indicate that YM-11170 is a very potent inhibitor of gastric acid secretion and that twice daily medication of 20 mg YM-11170 is recommendable for further antisecretory studies with ulcer patients.