This study evaluated potential contributions of daunorubicin and its principle metabolite, daunorubicinol, to the cardiotoxicity of daunorubicin therapy. Daunorubicin (15 mg/kg) or placebo (normal saline) was administered by iv bolus to New Zealand white rabbits and 3 to 4 days later, hearts were removed to measure contractility (dF/dt), concentrations of daunorubicin and daunorubicinol, and evidence of oxidative stress on glutathione and glutathione peroxidase. Contractile function of isolated atria and papillary muscles was depressed (p < 0.05). Daunorubicinol exceeded daunorubicin concentration in the heart (p < 0.005) with a ratio of metabolite to parent drug of 26 in atrial and 32 in ventricular tissue. There was a significant correlation between peak plasma (r = -0.63; p < 0.05) or cardiac concentration (r = -0.78; p < 0.02) of daunorubicinol, but not daunorubicin, and depression of dF/dt in papillary muscles. In separate in vitro studies, daunorubicinol at a concentration (5.5 micrograms/g tissue or 10 microM) approximating that observed ex vivo in heart inhibited Ca2+ uptake into cardiac sarcoplasmic reticulum vesicles by 39 +/- 3%, whereas 10 microM daunorubicin (14-fold higher than actual ex vivo cardiac concentrations) did not demonstrate any detectable inhibition. Daunorubicin treatment failed to significantly alter concentrations of GSH or GSSG or activities of glutathione peroxidase in the heart. Thus, cardiac dysfunction observed 3 to 4 days after a single dose of daunorubicin did not clearly relate to oxidative stress, but was associated with a cardiac concentration of daunorubicinol that appeared sufficiently high to impair Ca2+ metabolism.