We investigated whether elemental diet feeding would protect the intestine from radiation injury. Five dogs were fed an elemental diet for three days before receiving pelvic irradiation (500 rad/day for four days) and were maintained on the diet during the days of irradiation. These dogs were compared with five dogs that were fed normal kennel ration, but were treated similarly otherwise. One day and five days following completion of the radiation treatment, the dogs were anesthetized and a biopsy specimen of terminal ileum was taken for histologic and electron microscopic studies. In the dogs fed the elemental diet, there was no significant damage to the intestine seen on histological examination, and electron microscopy disclosed elongated microvilli and no organelle damage. However, both histological and electron microscopic examination of the intestine from dogs maintained on normal kennel ration showed that severe damage had occurred from the irradiation procedure. It seems, therefore, that the feeding of an elemental diet to dogs as a prophylaxis can afford protection to the intestine from the acute phase of radiation injury.