Mice received 4' Epi-adriamycin (4' Epi-ADM) i.p. in increasing doses. After 48 hours, hearts were removed and mitochondria were isolated. 4' Epi-ADM had no effect on the enzymatic activities of complexes I-III and complex IV of the mitochondrial respiratory chain; no modification of the mitochondrial membrane viscosity was observed and only a slight membrane lipid peroxidation was measured. On the contrary, in vitro studies showed a 4' Epi-ADM dependent enzymatic inactivation of complex I-III and IV, correlated with a mitochondrial membrane rigidification and an enhanced lipid peroxidation rate.