These experiments show that the common laboratory animals are about equally sensitive to the x-ray given over the abdomen. The clinical reaction following a M.L.D. is very similar and the intestinal pathology almost identical. The rat and guinea pig are slightly more sensitive to the x-ray than are the dog, cat, and rabbit. By contrast birds, frogs, and reptiles are very resistant to the x-ray and may tolerate two or three doses of radiation lethal for dogs. We can offer no convincing explanation for this fact which is discussed above. These data strengthen our belief in the scattered and incomplete observations on human cases which indicate that the human intestinal tract is likewise sensitive to radiation. This fact must be given careful consideration in conditions where abdominal or pelvic radiation is being used because such injury done to intestinal epithelium is always serious and in some cases irreparable.