Omeprazole in doses of 20, 40 or 60 mg dose-dependently inhibits basal acid secretion in young healthy subjects, with almost complete inhibition by 60 mg. Omeprazole has also been shown to dose-dependently inhibit vagally stimulated and meal stimulated acid secretion. Pentagastrin stimulated acid secretion is strongly inhibited by omeprazole for 4-5 hours, with moderate inhibition remaining 24 hours after a single dose. The plasma half-life of omeprazole, however, is about 50 minutes. The inhibitory effect accumulates over the first few days of repeated administration, and the effect continues for at least 24 hours after the last dose. A relatively high dose of intravenous omeprazole is required to keep the gastric pH above 4 over the 24-hour period. Both basal and postprandial serum gastrin concentrations have been observed to increase during omeprazole treatment. These changes, however, are probably secondary to a pronounced reduction of intragastric acidity which relieves acid inhibition of gastrin release from the antrum.