The effect of ascorbate in reducing Adriamycin toxicity has been examined in mice and guinea pigs. Ascorbate had no effect on the antitumor activity of Adriamycin in mice inoculated with leukemia L1210, but it significantly prolonged the life of mice and guinea pigs treated with Adriamycin. Adriamycin elevated lipid peroxide levels in serum and liver, and ascorbate prevented the elevation. The significant prevention of Adriamycin-induced cardiomyopathy by ascorbate was proved by means of electron microscopy. The earliest alterations of dilation of the sarcoplasmic reticulum and transverse tubular system and the appearance of a large number of cytoplasmic fat droplets, which were seen in cardiac tissue from guinea pigs receiving Adriamycin, were apparently reduced in animals that were treated with ascorbate.