Ninety-six CD-1 male rats were exposed to γ-ray doses (0-25 Gy) in increments of 5 Gy. One femur, the surgically exteriorized GI tract, and the oral cavity were shielded during irradiation to protect against acute mortality from injury to the hematopoietic system, small intestine, and oral cavity. In addition, the thoraxes of half of the animals from each dose group were shielded. At approximately monthly intervals from 2 to 10 months after irradiation the hematocrit, plasma urea nitrogen (PUN), and${}^{51}{\rm Cr}\text{-}{\rm EDTA}$ clearance were measured. During the study 20 thorax-shielded and 19 thorax-irradaited animals died. All rats whose thoraxes received 25 Gy irradiation and three out of seven rats whose thoraxes received 20 Gy died 1 to 3 months postirradiation with massive pleural fluid accumulation. Shielding the thoraxes prevented this mode of death at these doses. Kidney injury was judged to be the primary cause of death of all thorax-shielded animals and 15- and 20-Gy thorax-irradiated animals. Animals with kidney damage had elevated PUN and reduced${}^{51}{\rm Cr}\text{-}{\rm EDTA}$ clearance and hematocrits. The relative merits of each of these end points in assessing radiation-induced kidney injury after total-body exposure are discussed.

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