The effect of glucagon on gastric acid and pepsin secretion, basal or stimulated by a meal, pentagastrin and histamine, was studied in duodenal ulcer patients. Intravenous glucagon infused in graded doses ranging from 6.2 to 50 mug per kg-hr produced a dose-related inhibition of pentagastrin-induced acid secretion reaching about 40% of the control level at the dose of 50 mug per kg-hr. Acid inhibition was paralleled by a decrease in the pepsin output and serum calcium level and was accompanied by a rise in the blood glucose concentration. Glucagon used in a standard dose of 25 mug per kg-hr produced about 50% inhibition of acid secretion induced by a meal (measured by intragastric titration) accompanied by a significant decrease in the serum gastrin level measured by radioimmunoassay. Histamine-induced secretion was only slightly inhibited by glucagon, and the degree of inhibition for acid (25%) and pepsin (20%) secretion was statistically insignificant.