343 white Wistar rats were divided into three groups of 113, 115 and 115 animals in each group. A control group received an injection of acetic acid into the wall of the antrum whereas the two other groups underwent first, in one group, a vagotomy and pyloroplasty, in the other, a pyloroplasty alone. In all the control animals there was observed, during the days following the injection, the existence of a severe antral ulcer. This ulcer improved with time upon 40th day, then around the 70th day appeared a second ulcer cycle. In the operated animals, the ulcers were definitively less frequent, and the course was linear without a second ulcer cycle. In the light of physiological and morphological evidence, it was shown that injection of acetic acid caused a submucosal lesion with a constant linear course towards the development of a callus which became hyalinised all the more quickly when it was submitted to endogenous agression by gastric juice. Above this callus, the gastric mucosa attempts to regenerate and, owing to the poor quality substratum, becomes ulcerated again, thus constituting the various cycles of peptic ulcer disease. Pyloroplasty and to a lesser degree, vagotomy-pyloroplasty, reduce stasis of the gastric juice and reduce endogenous agression of the callus and thus slow the unfavourable course.