In four healthy subjects and in 12 patients with duodenal ulcer (DU), graded balloon distension of the gastric fundus and body caused increments in acid output related to the distension volumes. The mean peak distension response amounted to about 50% of the mean peak acid output (PAOpg) evoked by intravenous infusion of pentagastrin in a dose of 300 microng/h, eliciting maximum observed secretory response. During distension with the largest balloon volume, 1-0 mg atropine injected intravenously significantly depressed the acid secretory rate, the median inhibition amounting to about 80%. In two patients with DU, 2-5 mg atropine completely abolished acid secretion during fundic distension. In nine subjects with DU, complete proximal gastric vagotomy profoundly depressed the secretory responses to graded fundic distension, eliminating the acid response to the smallest balloon volume used. A slight, but significant, response to the largest distension volume persisted after complete vagotomy. Incomplete proxomal gastric vagotomy was found to reduce only moderately the distension responses in five patients, and the peak acid response to fundic distension as related to PAOpg remained unchanged. In conclusion, distension of the gastric fundus and body in man stimulates acid secretion by means of an atropine-sensitive, presumably cholinergic, reflex mechanism and the findings after vagotomy are in agreement with the concept that this reflex mechanism is conveyed by both short intramural and long vagovagal pathways.