1. Serum gastrin concentrations before and after a standardized meal were determined in twenty-eight patients with duodenal ulcer and in ten normal control subjects. 2. In response to pentagastrin, thirteen of the duodenal ulcer subjects secreted acid within the limits of normal and fifteen secreted in excess. 3. The differences in the basal serum gastrin concentrations between the three groups, normal subjects, acid "normosecretors" and hypersecretors were not statistically significant but that of the hypersecretors was suggestively low. 4. The integrated gastrin response and peak gastrin responses to meals were higher in duodenal ulcer patients with normal acid secretion than in the hyper-secretors but the values for the latter were not different from normal subjects. 5. Stabilization of intragastric pH by infusion into the antrum of sodium bicarbonate during the test meal response period did not alter these differences between the two ulcer patient groups. 6. A significant inverse correlation exists between the maximal acid output and the integrated gastrin response in both normal subjects and hypersecreting duodenal ulcer patients. 7. The evidence (a) supports the existence of an inverse relationship between the functioning parietal cell and gastrin cell masses, (b) shows the gastrin response in normosecreting ulcer subjects to be inappropriately high, and (c) suggests that excessive vagotonia exerts trophic effects upon both parietal cell mass and gastrin cell mass.