White Leghorn roosters, 3-4 days after hatching, received single exposures to either60 Co γ-rays or 200 kVp x-rays. Doses were in or immediately below the acute lethal range. Survivors at 35 days were caged individually and maintained for duration of life observations (85 control and 193 irradiated birds). Survival time, weight, and major pathologic findings were determined in control and in four irradiated groups (A, 24-hr γ-ray exposure to 700-1000 R, 30-day lethality <3%; B, 24-hr γ-ray exposure to 1200-2000 R, 30-day lethality 10-80%; C, 6-60 min γ-ray exposure to 700-1100 R, 30-day lethality 10-80%; D, 30-min x-ray exposure to 1000-1200 R, accompanied by prophylactic treatments that reduced acute lethality from 99% to 30-80%). Mean survival time of controls was about 6 years and maximum life span was 10.6 years. A single γ-ray exposure in the acute lethal range (groups B and C) resulted in a mean life shortening of about 50%, and the decrease in body weight (growth) was correlated with decrease in mean survival time. When the exposure time was increased from 1 hr to 24 hr, radiation effectiveness for life-span shortening and for weight was reduced. In x-irradiated birds protected against acute lethality by various prophylactic treatments, there was a differential effect on survival and growth; growth was more severely reduced than would be expected on the basis of 30-day mortality or life-span shortening. The principal radiation-induced lesions were those characteristic of cardiac failure; these were observed most frequently in birds that died before 5 years of age. Tumors were commonly testicular in origin; they were found with equal frequency in irradiated and unirradiated groups. The incidence of leukosis, a virus-induced neoplasia, was reduced in irradiated groups and was zero after the higher radiation exposures (groups B and D).

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