A new and simple method of producing gastric (antral and fundic) and duodenal ulcers in rats by the topical application of acetic acid to the serosal surface is described. Three to 5 days after treatment, clearly defined, deep, round or oval ulcers, penetrating into the adjacent organs (mainly liver), appeared in the mucosal area corresponding to the treated area. The fundic and duodenal ulcers gradually diminished in size within 10–60 days, and, in all cases, were completely healed after 80 days, although the regenerated epithelium was superimposed on connective rather than on glandular tissue. In contrast, relatively large antral ulcers persisted for up to 80–100 days in more than 50% of the animals. A unique and characteristic histologic feature of the antral ulcer was the appearance of glandular cysts in the margin or base of the ulcerated area in the antrum. This experimental method appears suitable for screening therapeutic agents.