The effect of omeprazole on the overnight and pentagastrin-stimulated gastric secretion of acid and pepsin have been studied in healthy male volunteers. After treatment with omeprazole, 30 mg or 60 mg daily for one week, overnight secretion of acid was reduced by 48 and 73%, respectively. During stimulation with pentagastrin, a single dose of 40 mg omeprazole reduced acid output by 98%, while after 80 mg, secretion of acid and pepsin was completely abolished. Omeprazole is one of the most potent gastric secretory inhibitors available at present, with potential for use in the therapy of ulcer disease.