We studied the effect of intravenous administration of a gastrin-receptor antagonist (proglumide) on gastric acid secretion and serum gastrin in three patients with Zollinger-Eliison syndrome. Proglumide administered as a bolus injection (50 nig/kg) or as an infusion (50 mg/kg hour and 100 mg/kg hour) or as a combination of bolus injection and infusion (50 mg/kg I.V. followed by 50 mg/kg hour) inhibited gastric acid secretion by 13–62%. Cimetidine (2 mg/kg I.V. followed by 2 mg/kg hour) inhibited gastric acid secretion by 83–86%. Neither proglumide nor cimetidine significantly influenced serum gastrin concentrations. We conclude that proglumide is a relatively weak inhibitor of gastric acid secretion in patients with the Zollinger-Ellison syndrome.