Whole-body X-irradiation of rats caused inhibition of endocrine function of thymus. The effect was a function of radiation dose and time after irradiation. 72 h following irradiation with doses of 6 and 8 Gy the thymus hormone content of blood serum fell down the level registered in the thymectomized animals. Cellularity of the thymus and spleen concurrently decreased. The kinetics of spontaneous chemiluminescence of blood serum, thymus and spleen cells characterized the hypersecretion of glucocorticoids in response to radiation activation of lipid peroxidation in radiosensitive rat organs.