Optimal conditions for ulcer production in the pylorus-ligated mouse seem to be the following: the use of animals between 9 and 11 weeks of age, a preoperative starvation period of 15 hours, an experimental period of 8 hours, and an ambient temperature of 28℃. Under these conditions, ulcers are produced in nearly all animals. It appears that gastric acidity in ulcer formation in the mouse may be secondary to gastric distention and interference with circulation. Submitted on December 1, 1958