Abstract: Exposure of Sprague Dawley white rats to single doses of 900 kvp X radiation demonstrated a medium lethal dose of 600 r which was not markedly dependent upon sex, weight, or, in the case of females, stage of the estrous cycle at the time of irradiation. Deaths within 30 days were most frequent between the fourth and eighth, and between the tenth and fifteenth post- irradiation days, indicating the existence of at least two processes leading to death in the "acute" period.