The inhibitory effects of intravenous infusions of secretin, glucagon and caerulein on the gastric acid response to bombesin were studied in 8 duodenal ulcer patients. Bombesin was found to be a very potent stimulator of gastric acid secretion in patients with duodenal ulcer. There were no significant differences in acid outputs per 15-min period between bombesin infused in a dose of 0.9 μg/kg/h and pentagastrin infusion administered in a maximal dose, at a rate of 6.0μg/kg/h. Secretin (1 U/kg/h), glucagon (30μg/kg/h) and caerulein (0.1μg/kg/h) produced significant decreases in gastric acid secretion evoked by bombesin given in a dose of 0.9 μg/kg/h. Percentages of inhibition were 48.6, 45.2 and 35.5, respectively. It is supposed that secretin and glucagon given in pharmacological doses are capable of interfering with the action of gastrin released from antrum by means of bombesin on the parietal cell by noncompetitive kinetics. Caerulein administered in a pharmacological dosis, however, can inhibit the effect of gastrin released by bombesin on the parietal cells by a competitive kin