Several methods have been reported to produce an experimental chronic ulcer in the rat, such as thermal ulcer by Skoryna et al. (1, 2), clamping-cortisone ulcer by Umebara et al. (3), and methylcholanthrene ulcer by Lauren et al. (4). In these methods thermal and clamping-cortisone ulcers seem to be valuable tools in investigation of the healing process of gastric ulcer, because of the relatively long period of its persistence and closer histologic resemblance to human peptic ulcer. However, both methods include the undesirable procedure such as the incision of glandular portion of stomach in order to perform the thermocautery of the innersurface of stomach in the former method and the consecutive administration of cortisone acetate for 8 days after the gastric operation in the latter. On the other hand, Robert and Selye (5) have reported the method to evoke an ulcer by an injection of formalin solution into the rat stomach wall itself. We also tried the same procedure and found the gastric necrosis corresponding to the injected area. However, the necrotic tissue did not disappear from the stomach wall even 6 days after the operation so that the so-called gastric ulcer was not produced. Then we injected the acetic acid solution, which is well known to injure the gastric mucosa (6) or to evoke inflammation in rat paw in our laboratory (7), into the rat stomach wall and found the definite gastric ulcer having resemblances to the human peptic ulcer at gross and microscopical observation. This paper deals with the studies of the healing process of the produced ulcer and the usefulness for the assay method of the curative drugs of gastric ulcer.