Gastric acid secretion in response to food is measured by intragastric titration in normal human subjects and in patients with duodenal ulcer. Current projects include studies on the effects of secretory inhibitors (histamine H2 receptor antagonists, anticholinergic drugs), enterogastrones (secretin), and physiologic variables (pH, fat and protein content of the diet, etc.). The studies in normal subjects are directed towards increasing our knowledge of gastric physiology; the experiments in patients are aimed at clarifying the pathogenesis of peptic ulcer.