Abstract Using isolated atria, the positive inotropic action of digitoxin and digoxin was compared quantitatively with the cardiac action of metabolites found in guinea pigs. The bis- and monodigitoxoside of digitoxigenin and the monodigitoxoside of digoxigenin showed the strongest inotropic effect with respect to the affinity as well as the maximum response. The corresponding conjugation products did not alter the contraction amplitude.