The effect of an intravenous infusion of omeprazole (0.35 mg/kg) and placebo on basal and stimulated (pentagastrin 1.0 microgram/kg/h) secretion of gastric acid, intrinsic factor and pepsin was studied in 10 healthy male subjects. Omeprazole caused a marked inhibition of basal and stimulated acid output. The inhibition of pepsin output was less marked, but also significant. The output of intrinsic factor, however, showed no significant change. The results indicate that acid and intrinsic factor might have different secretory mechanisms within the parietal cell.