Effects of tathione and cepharanthin, the radiation-protective agents, on the lipid peroxidation in rat organs were studied, and obtained the results as follows. (1) The acceleration of lipid peroxide formation induced by X-irradiation can be inhibited by the administration of tathione prior to the irradiation and by the dose administered. Such an inhibitory effect is especially marked in the liver, and as reported in Part Ⅰ and Part Ⅱ, in view of correlations among X-irradiation effect, lipid peroxidation and fatty acid composition in the liver, it seems that glutathione in vivo is somehow involved in the acceleration of lipid peroxidation. (2) The increase of lipid content observable in the liver of X-irradiated animal could not be observed in the animals administered with tathione. (3) Cepharanthin showed not any relation to the lipid peroxidation.