The histologic changes induced by irradiation of the stomachs of dogs were examined one hour to nine months after irradiation. The irradiated area of the stomach progressed to ulcer formation by way of hyperemia and erosion; on the other hand, there were simultaneous signs of recovery. Regenerating epithelium appeared during the first week and migrated from the surface adjacent to the ulcer to the center of the ulcer. Subsequently the muscularis mucosae extended, and then the submucous coat was repaired. Regeneration of the nerve fibers around the muscularis mucosae seemed to reach its maximum three weeks after irradiation. These proliferating nerves degenerated with deepening of the radiation ulcer, but they were unexpectedly radioresistant and protrude from the surface of the ulcer into the necrotic surrounding tissues. It is concluded that the effect of irradiation on the alleviation of the pain of stomach cancer is not due to the degeneration of the nerves but to elimination of the stimulation of nerves on account of the radiation effect on the cancer cells themselves.