Experimental gastric ulcers, produced by submucosal injection of acetic acid in rats were studied in relationship to healing. Small ulcers healed completely within 40 days after induction; moderately severe ulcers were sometimes found at 150 days, and probably resulted from repeated healing and reulceration. Large severe ulcers partially healed and then became reexacerbated; they frequently increased in size up to 250 days, thus resembling chronic gastric lesions. Throughout the experimental periods, adhesion of the ulcer base with adjacent organs and delayed gastric emptying were found in most animals. After ulceration, an increase in gastric volume and acid output and a decrease in protease activity of gastric contents were found in chronic fistula rats with experimental ulcers. The secretory changes observed here may have been secondary to gastric ulceration and/or delayed gastric emptying. The factors responsible for the reexacerbation of the experimental ulcer remain unidentified.