Adriamycin is an anthracycline drug with a wide spectrum of clinical antineoplastic activity. However, the usefulness of the drug is limited by its dose-dependent cardiotoxicity. Adriamycin-stimulated free radical formation has been suggested as one of the mechanisms for its cardiotoxic effects. In order to evaluate this underlying mechanism, we have perfused rat hearts with Adriamycin, using a modified Langendorf technique, and the free radicals formed were analyzed by electron spin resonance spectroscopy using spin-trapping techniques. Our studies show that Adriamycin stimulated the formation of ·OH in the heart, and the maximum ·OH was formed with 1 µm of the drug. The addition of superoxide dismutase (600 units/ml) inhibited the hydroxyl radical formation by 2-to 3-fold, while catalase (550 units/ml) abolished it completely, showing the intermediacy of superoxide and H2O2. Furthermore, ICRF-187, an iron chelator and a cytotoxic drug, was also an effective inhibitor of .OH formation in the rat heart. The heart rate was not significantly modified by all the above experiments. This study demonstrates that Adriamycin stimulates the formation of ·OH in the isolated rat heart and suggests that this mechanism may be significant in Adriamycin-induced cardiotoxicity.