Urinary bladder augmentation with a segment of the stomach, i.e., gastrocystoplasty, has been used to improve capacity and compliance in patients with bladder dysfunction. In the present study, rats were subjected to gastrocystoplasty (using the oxyntic segment) with or without fundectomy (removal of the oxyntic part of stomach), and the acid secretion in the augmented bladder was measured. In freely fed rats, the pH values were neutral and not significantly decreased in the rats subjected to gastrocystoplasty with or without fundectomy compared to controls (no operation or sham operation). In response to food intake after being fasted, the rats subjected to gastocystoplasty + fundectomy produced significant amounts of acid. Immunohistochemical examination revealed that the ECL cells and parietal cells seemed to be normal in rats with gastrocystoplasty alone, and that micronodules of ECL appeared to develop in rats with gastrocystoplasty + fundectomy. We suggest that the rats subjected to gastrocystoplasty + fundectomy are capable of producing acid secretion in the bladder, probably due to the secretagogue and trophic effects of gastrin on the ECL cells in the segment of the oxyntic mucosal segment of the bladder.