The effects of whole-body irradiation with lethal and sub-lethal dose (1000 and 600 R) of X-ray on the adrenal function of rats were studied by estimating corticosterone and sterol biosynthesis from ¹⁴C-acetate in vitro to assess a functional state of adrenals. At 24 hours post-irradiation, the elevated corticosterone biosynthesis was observed in the adrenals obtained from both 600 and 1000 R X-irradiated rats, while the accelerated incorporation of ¹⁴C-acetate into free sterol was noticed in 600 R irradiated adrenals but not in the 1000 R irradiated. At 48 hours after irradiation, the elevated function of 600 R irradiated adrenals was maitained, while in the 1000 R irradiated adrenals, the both corticosterone and sterol formation in vitro were observed to be depressed markedly. The relationship between lethality and adrenal function in irradiated animals was discussed.