An experimental gastric ulcer was produced in a dog by administering x-rays (110 K.V.M., 5 ma. without filter) for 1/2 hr. to the mucosa of the posterior wall of the stomach, exposed by inversion of the walls through a slit made longitudinally immediately proximal to the pyloric sphincter. The affected area, 3 cm. diameter, was limited by lead-plates. The ulcer thus formed lasted in 1 dog for 585 days, but in others perforation occurred, or, after 411 days, healing. Neither hypersecretion nor hyperacidity followed the development of the chronic gastric ulcer. Stomach motility varied with the position of the ulcer; if 5 cm. away from the pyloric sphincter, no effect; if nearer, motility is decreased.