Maximal gastric secretion was studied in 41 unoperated patients with gastric ulcer, 201 patients with duodenal ulcer, and 122 control subjects. The patients with a gastric ulcer were divided into high, body and prepyloric, according to the site of the ulcer. Both high and body gastric ulcers secreted significantly less than the controls, and the duodenal ulcer patients secreted significantly more. However, the patients with gastric ulcer were older and lighter than the controls and, since these factors are known to influence maximal gastric secretion, the controls and patients with duodenal ulcer were standardized to the mean weight (and age, for the controls) of the gastric ulcer group. After this standardization, there was no significant difference in secretion between the patients with body ulcers and normal controls. Pyloric loss was a similar percentage of maximal gastric secretion in all groups, but duodenogastric reflux was higher in the gastric ulcer group compared to the controls. The mean volume of duodenogastric reflux was greatest in the patients with a prepyloric gastric ulcer. It is commonly accepted that hyposecretion in patients with a gastric ulcer is due to gastritis consequent upon increased duodenogastric reflux. However, in this study, no gastric hyposecretion was evident in the body and prepyloric gastric ulcer groups, both of whom had greater than normal levels of duodenogastric reflux.