208 ATOMIC MEDICINE As the dose is increased above the LDioo the first peak of deaths becomes progressively more prominent and the second peak fades out so that with doses of 1500 to 10,000 r the mean survival time is approximately 4 days. A similar phenomenon is seen with rats; however, the first peak is more prominent at lesser doses of radiation. The first peak of deaths has been correlated with severe gastrointestinal injury and dysfunction (Bond et al.; Quastler ef al.; Brecher and Cronkite; Cronkite and others). The relative sensitivities of the species differ, the rat GI tract being relatively more sensitive. The second peak of deaths is correlated with the sequelae of pancytopenia Gnfection, hemorrhage, and anemia), If the LDso of the species is low so that amounts of radiation are less than that needed to produce the severe GT injury and dysfunction, the species has essentially a unimodal distribution of deaths in the lethal range, with a meansurvival time of approximately 10 to 15 days, e.g., the dog. If LDso is high, a modal distribution of deaths may appear, e.g., mouse, rat and rabbit. Where man fits into the above relative sensitivities of tissue is not yet known; however the evidence indicates that man responds in a manner similar to the dog. The early diarrhea (first 4 days) is correlated with direct radiation injury of the gastrointestinal tract; however, late diarrhea (7 to 24 days) occurs as a result of ulecrations and hemorrhage due to the pancytopenia. Studies strongly indicate that complete histological recovery of the bowel oecurs by 4 to 5 days, hence if the animals survive to the stage of Jate diarrhea, the bowel has been reconstituted. Histological studies show typical. hemorrhagic and agranulocytie Jesions in a bowel that is otherwise approximately normal in appearance (Conardet al. and Brecheret ai.). With the preceding in mind, the following is formulated with regard to the probable response of man to penetrating radiation exposure at different dose levels. With small amounts of total-bedy radiation (under 100 to 150 r), there may be no symptoms or at most a transient nausea. Leukopenia, particularly the lymphocytopenia, will be mild and of short duration. With larger amounts of radiation, 150 to 1000 r, the characteristic clinical picture may develop. Within a few hours, pronounced nausea, vomiting, malaise, weakness, headache, dizziness, anorexia, tachycardia, irritability and insomnia will generally appear. Leukopenia, anemia and thrombocy- topenia will develop at different rates (Cronkite; Lawrence, Dowdy and Valentine; Jacobson ef al.; LeRoy). The symptoms will usually subside within 24 to 48 hours to appear again after a few days. The interval between the initial and the subsequent symptoms has been termed “the latent period.” This latent period will become shorter with larger doses and may be absent if the dosage is sufficiently high. With termination of the la-