Adriamycin toxicity is postulated to result from cytochrome P-450 reductase-catalyzed univalent reduction of the quinone to the semiquinone free radical intermediate. Oxygen radicals generated during the nonenzymatic reoxidation of the semiquinone have been implicated in the deleterious modification of a variety of tissue macromolecules. Detoxification of reactive products, such as hydroperoxides, is proposed to involve the consumption of vital cellular reducing equivalents which may, in itself, represent the primary causative event in toxic tissue damage. The present investigation demonstrates that hepatic tissue has sufficient glutathione (GSH) reductase to prevent a decrease in GSH following acute adriamycin administration to rats. Similarly, except for a transient decrease in NAD, adriamycin intoxication caused minimal changes in the hepatic pyridine nucleotide content in vivo. It is concluded that species- and tissue-specific differences in the distribution of antioxidant defense mechanisms may be primary determinants of the relative insensitivity of liver and, in contrast, the rather selective cardiomyopathy resulting from adriamycin administration in vivo.