The dose response curves for acid and pepsin output to increasing intravenous doses of pentagastrin (0.01, 0.02, 2, 8, and 16 micrograms/kg per hour) were determined in six male patients with duodenal ulcer and six with type 1 corporeal gastric ulcer before and 3 to 6 months after selective proximal vagotomy and excision of the gastric ulcer. The maximal secretory capacity (maximal response) of acid and pepsin was greater in the duodenal ulcer patients than in the corporeal gastric ulcer patients, but the sensitivity of the oxyntic and peptic cells to pentagastrin (the dose required for half the maximal response) was equal for the two ulcer groups. Selective proximal vagotomy reduced the acid response to insulin by 96 to 100 percent. The acid secretory capacity and the sensitivity of the oxyntic cells to pentagastrin was reduced by selective proximal vagotomy to the same extent in the duodenal ulcer patients and the corporeal gastric ulcer patients. Selective proximal vagotomy reduced the pepsin secretory capacity in the duodenal ulcer patients but did not reduce the already low capacity in the corporeal gastric ulcer patients. Selective proximal vagotomy decreased the sensitivity of the peptic cells in both ulcer groups. Similar results were obtained when the dose response curves were analyzed according to Michaelis-Menten kinetics. Our results justify clinical trials of selective proximal vagotomy with complete ulcer excision for treatment of type 1 corporeal gastric ulcer.