The present study was conducted histologically to elucidate the influence of vagotomy on the gastric mucosa. Bioptical tissues were obtained from 10 control cases and from 26 cases of duodenal ulcer using an endoscope. Of the 26 cases of duodenal ulcer, 20 were subjected to truncal vagotomy with pyloroplasty and 6 to truncal vagotomy with hemigastrectomy. In the cases of duodenal ulcer, tissues were obtained at the preoperative stage, and at 1, 3, 6 and 12 months on the time-course basis after the operation. The parietal cells were counted in a unit area (0.0176 mm2) between the middle and lower 1/3 part of the fundic gland in H.-E. stained paraffin sections under microscopy. The length of the fundic gland was also measured. The results are summarized as follows: (1) There were a greater number of parietal cells and the length of the fundic gland was longer in the preoperative cases than in the controls. (2) Postoperative observations revealed that the number of parietal cells and the length of the fundic gland significantly decreased up to 3 months after the operation by both surgical techniques. Thereafter, the number of parietal cells and the length of the fundic gland significantly increased in the cases subjected to vagotomy with pyloroplasty. (3) The difference between the two surgical techniques was made clear when the mean values for the absolute number of parietal cells were compared at 3 months after the operation, the absolute number of parietal cells being smaller in hemigastrectomy than in pyloroplasty.