For the purpose to elucidate the formation and the mechanism of lipid peroxidation products in the serum following irradiation, rabbits were irradiated once withe 1000R over the whole body, and serum lipid proxidation products as well as proxidation products in each lipid fractions were measured as TBA level. In addition, a study was made on the correlation between the peroxidation product and the change of fatty acids, and the following results were obtained. 1) The whole lipid content of serum: The whole lipid content was found to have increased about 2.6 times that before the irradiation by 24 post-irradiation hours, and even after the lapse of 48 hours such a tendency persisted. 2) Serum whole TBA level: By 24 post-irradiation hours the whole TBA level increased markedly up to about 6.5-fold that before the irradiation. 3) Lipid content of fraction: Except for cholesterol, every lipid fractions are found to have increased after irradiation. Especially marked is the increase of triglyceride. 4) TBA level of fractionated lipid: There is seen a marked increase of cholesterol ester, which proctically occupied entire serum TBA value. Next marked was the increase of phospholipid, and quantitatively it was classified that the increases seen in triglyceride and free fatty acids are not concerned with the rise in the free fatty acid content and TBA level. 5) Serum lipid contents and TBA level infasting: By taking the level of serum lipid 24 hours after the start of fasting as one, the serum lipid levels were studied at 48 and 72 hours after the start of fasting, and it was found that both serum lipid and TBA levels rose only very slightly. 6) Changes of fatty acids: The relative ratio of palmitic acid of the whole fatty acids increased after irradiation, and the ratio of linolic acid and linolenic acid was decreased by irradiation while by 48 hours the relative ratio of linolic acid was decreased to about 1/5 that before irradiation, and the relative ratio of linolenic acid was markedly decreased to about 1/35.