In 29 patients with duodenal ulcer, suction biopsies were obtained from the body of the stomach within 2 weeks before and 3 and 12 months after proximal gastric vagotomy (PGV). Only those with a negative insulin test were included in the study. Three months after PGV basal and pentagastrin-stimulated acid secretion was reduced by 82 and 64 percent of preoperative values, respectively. Twelve months after the operation the corresponding figures were 63 and 56 percent. There was a similar acid reduction in patients with or without gastritis. The number of patients with atrophic gastritis increased from 3 before the operation to 11 3 months after PGV (p smaller than 0.01). There was no further increase from 3 to 12 months postoperatively. The average thickness of gastric mucosa and the mean number of parietal cells per unit area before PGV did not show any significant change 3 months and 1 year after the operation. There was a significant correlation between the number of parietal cells per unit area and maximal acid output before (p smaller than0.01) but not after PGV. Acid output per parietal cell was higher before PGV, suggesting that the decrease in gastric secretion after PGV cannot be attributed to the development of gastritis or reduction of parietal cell number.