The disposition of [14-14C]4-demethoxydaunorubicin HCl ([14-14C]idarubicin HCl, [14C]IDR) and of [14-14C]daunorubicin HCl ([14C]DNR) was studied in male Sprague Dawley rats. [14C]IDR was administered either IV at 0.25 mg/kg body weight or PO at 1 mg/kg body weight, whereas [14C]DNR was dosed IV at 1 mg/kg body weight. The main elimination route for both compounds was the bile, fecal excretion representing 0.75-0.8 times the total dose at 72 h. Radioactivity due to [14C]IDR-derived species is released by the tissues at a slower rate than activity derived from [14C]DNR. After IV treatment comparable plasma levels are obtained, but tissue radioactivity is markedly lower with [14C]IDR, in keeping with the lower dosage. The ratio of plasma to tissue radioactivity is even higher in animals treated PO with [14C]IDR, because of the more extensive metabolism after this route of administration. The 13-dihydro derivatives of both [14C]IDR and [14C]DNR are the main metabolites in tissues, but in the case of the former, products of phase II reactions become more important at later times in liver and kidney and in excreta.