The effects of omeprazole, a substituted benzimidazole, on gastric acid and pepsin secretion have been studied in twelve healthy subjects. From six to eight hours after a single oral dose of 30 mg, there was a 66% reduction in basal acid output, and a 71.2% reduction in pentagastrin stimulated acid output. A single dose of 60 mg produced a 91.7% reduction in basal acid output and a 95.3% reduction in pentagastrin stimulated acid output. After seven days treatment with 30 or 60 mg daily, there was almost 100% inhibition of both basal and pentagastrin stimulated acid output. Omeprazole did not significantly affect pepsin secretion which is in keeping with its proposed mode of action, as an inhibitor of the H+/K+-ATPase enzyme on the secretory membrane of the parietal cell. There were no side effects after omeprazole either with single or repeated dosing.