The gastric mucosa secretes both acid and bicarbonate. In patients with duodenal ulcers, gastric acid hypersecretion is well-established. However, gastric bicarbonate secretion rates in these patients have not been reported. Accordingly, we calculated gastric acid and bicarbonate secretion rates in 13 patients with duodenal ulcer and 13 healthy subjects (controls) using a recently validated method that uses measurements of gastric juice volume, acidity (hydrogen-ion concentration), and osmolality. Non-parietal and parietal gastric volume secretion were also calculated. Although ulcer patients secreted significantly more acid than controls under basal conditions and during a submaximal intravenous pentagastrin infusion, they secreted similar amounts of bicarbonate. Ulcer patients secreted significantly more parietal and nonparietal fluid than controls. Increased nonparietal secretions in duodenal ulcer patients diluted excessive, acidic parietal secretions, preventing significant differences in gastric acidity in ulcer patients and control subjects (50.1 +/- 8.7 vs. 44.0 +/- 9.7 mmol/L basally; 111.0 +/- 3.1 vs. 102.4 +/- 7.5 mmol/L during pentagastrin infusion, respectively). These studies indicate that gastric bicarbonate secretion is close to normal in patients with duodenal ulcer, whereas acid secretion from parietal cells and nonparietal volume secretion are greater than normal.