To delineate stomach-pancreas interactions as related to the pathogenesis of duodenal ulcer, a comparison of gastric secretory function and exocrine pancreatic secretory function was made in controls, patients with gastric ulcer and those with duodenal ulcer. Gastric juice was aspirated through a Rehfuss tube before and after an injection of AOC-tetragastrin (4 μg/kg of body weight). Exocrine pancreatic function was determined by measuring the initial HCO(3) concentration (wash-out phase), HCO(3) output and maximal HCO(3) concentration of pure pancreatic juice collected endoscopically after intraduodenal instillation of hydrochloric acid (0.1 N, 100 ml). Serum secretin was also measured serially. Patients with duodenal ulcer showed significantly higher gastric acid secretion. They also showed significantly higher pancreatic HCO(3) output as well as higher initial HCO(3) concentration as in my previous study on exogenous secretin stimulation. A significant correlation was noted between gastric acid secretion and pancreatic HCO(3) secretion. However, the serum secretin response did not differ significantly among the three groups. These results indicate that patients with duodenal ulcer display a normal secretin release response to intraduodenal acid load and an augmented pancreatic bicarbobate secretion in response to the released secretin. These findings were discussed in relation to the pathogenesis of duodenal ulcer.