Acute duodenal ulcer can be produced in the rat by subcutaneous or oral administration of cysteamine. Development of this ulcer on the intestinal mucosal surface was followed from precursory stages to perforation using low power semimacroscopical and conventional histological techniques. The process started in the epithelium of the apical parts of the villi where segments of surface epithelial cells exfoliated and the lamina propria seemed to retract. As the process continued the villi gradually became lower eventually forming an avillous surface. Subsequently defects in the epithelium, necrosis, inflammation of the lamina propria, and a localized ulcer developed.