The effect of glucagon on basal and food-stimulated gastrin release, measured by radioimmunoassay, was studied in normal persons, patients with duodenal ulcer, patients with the Zollinger-Ellison syndrome, and Heidenhain pouch dogs. Intravenous glucagon (30 μg per kg per hr) diminished the basal serum gastrin values and the gastrin response to a standard meal in normal persons, in duodenal ulcer patients, and in dogs. In patients with the Zollinger-Ellison syndrome, glucagon caused an immediate increase in basal serum gastrin concentrations, and did not diminish the gastrin response to a meal. Lowering the glucagon dosage to 0.5 μg per kg per hr did not influence the inhibitory effect on basal values of gastrin or on stimulated gastrin release in normal persons. The effects of glucagon on gastrin are not related to the hyperglycemic effect of glucagon. These effects of glucagon on serum gastrin closely parallel the effects of secretin.