Omeprazole, a gastric proton pump inhibitor, is a substituted benzimidazole with a completely new mechanism of action that is, inhibition of the H+K+ATPase in the secretory membrane of the parietal cell. Several studies have shown a marked and long-lasting inhibition of gastric acid secretion after oral administration of the drug. During repeated once daily administration of omeprazole, the inhibition of pentagastrin stimulated acid secretion increases during the first few days of treatment and then stabilizes after 3–5 days. No information is available on pepsin secretion during repeated administration of omeprazole. The aim of this study was to investigate the dose-response relationship of once daily omeprazole treatment on acid and pepsin secretion.