After the whole body X-irradiation (650R) of rats, the lipid peroxide formation of various organs were estimated by the TBA reaction, and the results are presented as follows: (1) In TBA reaction tests on the lipid peroxidation in the liver, heart, spleen and kidney with homogenates being incubated, the TBA value were slightly high with endogenous homogenates, but on the addition of Fe(++) the reaction was further accelerated markedly. With liver and kidney that showed a high reaction on addition of Fe(++), the reaction was similarly accelerated on addition of ascorbic acid, but with heart and spleen the reaction at initial stage did not show any significant difference from that of the group without any addition. (2) In every organ from the irradiated animal the activity of lipid peroxide formation was greater than that in organs from non-irradiated control animals. (3) TBA value in the group added with Fe(++) were higher than those in the group without any addition, suggesting that X-irradiation greatly the lipid peroxidation in organs. (4) Change in the acceleration of lipid peroxidation accompanying the irradiation appears as a shortening of reaction-induction time. (5) Radioactivity-dose dependent change can be observed only in the liver, and the time-lapse change after irradiation can also observed only in the liver. (6) These results suggest that the lipid peroxidation following the whole body X-irradiation shows a significant change primarily in the liver.