The effects of lethal total-body x irradiation on various plasma lipid fractions and the degree of peripheral lipid mobilization were investigated in rabbits. Peripheral lipid mobilization was measured by the decrease in lipid concentration of a characteristic fat depot and the plasma free fatty acid response. Plasma phospholipid, free and ester cholesterol, and triglycerides were significantly increased after x irradiation. However, plasma free fatty acids were unaltered. Finally, a similar degree of lipid depletion from the adipose depot was noted in both control and x-irradiated animals, denoting that increased peripheral lipid mobilization was not a factor in the etiology of postirradiation hyperlipemia.