Roentgen radiation in lethal dosage, given over the abdomen of a normal dog, is followed by a physiological reaction of remarkable uniformity. The first 24 hour period following the exposure is negative clinically and anatomically, but histologically we see frank changes in the bone marrow, spleen, lymph glands, and ovaries. There are definite nuclear changes with degeneration in the crypt epithelium of the small intestine. The second 24 hour period shows slight clinical disturbances of gastrointestinal nature (vomitus and diarrhea). The mucosa of the small intestine shows scattered ecchymoses, but the histology of the small intestine is important. The necrosis of the crypt epithelium may be almost complete, while the epithelium of the villi remains practically intact. There are a little edema and invasion of wandering cells. The third 24 hour period shows increasing clinical disturbance with vomiting and bloody diarrhea. Anatomically the small intestine from the edge of the pylorus to the rim of the ileocecal valve looks raw, red, and inflamed. The crypt and villous epithelium has in large part vanished, leaving a collapsed framework of the mucosa showing a little edema and invasion of wandering cells. The 4th day marks the peak of the intoxication, and death usually takes place at this time, preceded by coma. Anatomically and histologically the picture is like that of the 3rd day. There is more evidence of mitosis and efforts of repair on the part of the intestinal epithelium. The stomach is not concerned in this reaction, but the colon may show evidences of a slight injury. The colon is obviously much more resistant than is the small intestine. We believe the evidence is conclusive that the injury done the epithelium of the small intestine is wholly responsible for the stormy clinical picture and fatal intoxication.