Antioxidants in organs of normal and irradiated rats were determined by means of the stable free-radical α,α-diphenyl-β-picrylhydrazyl. High doses of ionizing radiation reduced the level of water-soluble antioxidants; the fat-soluble antioxidants were unaffected, and the ability of the proteins to react with the stable free radical was unchanged or slightly elevated. It is concluded that the water-soluble non-protein antioxidants act as free radical scavengers in radiation. No great amounts of lipoperoxides were found in the irradiated animals. The basic analogy, but at the same time the great dissimilarities, between free radical processes in radiation and in autoxidation is discussed.