Changes in both the concentration of non-esterified fatty acids in serum and white (epididymal) adipose tissue, serum glycerol, and basal and L-noradrenaline-stimulated release of NEFA and glycerol from white adipose tissue in vitro were monitored 1, 6, 24, 48 and 72 h after a single whole-body irradiation of rats with a lethal X-ray dose of 14.35 Gy (1 500R). The NEFA concentration in serum and white adipose tissue was higher 1 h after irradiation as compared with that of sham-irradiated (control) animals; the free glycerol concentration in serum of the irradiated rats was lower than that of control rats. Basal release of NEFA was higher 1 h after irradiation, and lower 6 and 72 h after irradiation; the simulated release of NEFA did not change significantly. Basal and stimulated release of glycerol from the adipose tissue of irradiated rats was higher 6 h after irradiation. The relation between the changes in lipolysis and lipid mobilization and the changes in serum and tissue lipids in the irradiated organisms are discussed.