Gastroesophageal sphincter pressure and serum gastrin concentration were determined in the fasting state and after the intake of a protein food in 6 normal subjects, 6 patients with gastric ulcer, and in 6 patients with duodenal ulcer. No significant differences in the fasting state were found. After the food intake, gastroesophageal sphincter pressure increased significantly over basal values in normals and in patients with duodenal ulcer, but in patients with gastric ulcer a decrease in pressure was noted. Serum gastrin rose in all subjects studied after the food stimulation, but it was significant only in the gastric and duodenal ulcer group. In two normals and two patients with duodenal ulcer the ingestion of a potato meal of similar weight to that of the protein meal showed no change either in serum gastrin or in sphincter pressure. In one additional normal subject and one duodenal ulcer patient the constant intravenous infusion of Aminosol for 2 h produced no change in serum gastrin or sphincter pressure. These results indicate that the effect of protein food on sphincter pressure is different for gastric or duodenal ulcers, and, furthermore, that this effect is mediated by proteins in the gastrointestinal tract.