Gastric mucosal erosions were produced in rats by pyloric occlusion and intraperitoneal injection of epinephrine. Gastric ulcer was induced by application of acetic acid to a limited area of the stomach. pH, total amount of acid and volume of gastric juice were determined 5 h after pyloric occlusion. The presence of an acute acetic acid ulcer partly inhibited the development of mucosal erosions induced by pyloric occlusion and epinephrine. The gastric ulcer was associated with decreased acid secretion. A positive correlation was obtained between the degree of mucosal erosions and the total amount of acid in the stomach.