Comparative studies on gastric secretory function and exocrine pancreatic secretory function were made in 9 normal controls, 6 patients with gastric ulcer and 10 patients with duodenal ulcer by analysis of gastric juice and pure pancreatic juice (PPJ). Gastric juice was obtained through a Rehfuss tube at 10-minute intervals for 60 minutes before and after AOC-tetra-gastrin stimulation (4 μg/kg). PPJ was obtained by endoscopic retrograde catheterization of the papilla at one-minute intervals for 20 minutes after a bolus intravenous injection of secretin (Eisai, 1 U/kg) and for 10 minutes after an injection of CCK-PZ (Boots, 1 U/kg). Patients with duodenal ulcer had significantly high basal acid output (BAO) and maximal acid output (MAO), a significantly high bicarbonate concentration in the initial fraction of PPJ, and significantly high bicarbonate output during the secretin phase. A significant correlation was noted between the bicarbonate concentration in the initial fraction of the PPJ and the BAO and MAO, and also between the bicarbonate output and the BAO and MAO. These results indicate that bicarbonate secretion of the pancreas in response to secretin is proportionally raised in patients with duodenal ulcer to neutralize the raised gastric acid secretion. These findings were discussed in relation to the pathogenesis of duodenal ulcer.