With four groups of rats, normal, irradiated, fasting, and irradiated-then-fasting rats (dose of X-ray, 650R), 48 hours afterwards, liver, heart, spleen and kidney were examined separately as to changes in the acceleration of lipid peroxidation and fatty acids due to irradiation were examined and obtained the results as follows. (1) The lipid peroxidation in each organ was accelerated after irradiation when compared to that in the organs of normal group. In the fasting group the acceleration of lipid peroxidation was observed, but the acceleration in the irradiated group was far more marked, indicating clearly the effect of irradiation. (2) As to changes in the ratio of fatty acid composition in every organ of the irradiated group there were observed an increase in the ratio of high unsaturated fatty acids and a decrease in the ratio of saturated fatty acids as compared with the respective values in normal group. Among organs there could be recognized no uniform change in the fasting group and the irradiated-then-fasting group. These results indicate that the fasting affects the ratio of fatty acid composition, especially marked was change in triglyceride fraction. (3) The lipid content in the liver tends to increase after irradiation, and there is also observed an incrase in the ratio of lipids to phospholipids. (4) The arachidonic acid of phospholipid what is considered to be substrate of lipid peroxidation is increased in the irradiated group, suggesting its relation to the acceleration of lipid peroxidation accompanying irradiation, and actually, the arachidonic acid ratio of phospholipid is markedly decreased after the incubation of lipid peroxidation reaction in the presence of Fe(++), indicating that this substance can play the role of substrate.