Daunorubicin and adriamycin, anthracycline antibiotics, have been investigated by polarography, cyclic voltammetry, and chronopotentiometry in aqueous media at 37.5°C, as have their aglycones daunomycinone and adriamycinone. A mechanism is proposed to explain the complex reduction behavior observed. The significance of the mechanism is discussed in relation to the antineoplastic and cardiotoxic activity of the clinically useful daunorubicin and adriamycin.