The results of the studies reported here demonstrate the cardiac non-haematopoietic effect of erythropoietin, providing a new physiological function of the hormone. We demonstrate that myocardium from rat with chronic renal failure (CRF) showed an abnormal reponse to ouabain associated with an inhibition of cardiac Na<sup>+</sup>/K<sup>+</sup>/ATPase activity and with a decrease in the high affinity <sup>3</sup>H-ouabain binding sites. The extent to which both actions were improved with the recombinant human erythropoietin (rHuEpo) treatment suggests that the lack of the hormone is responsible for this phenomenon. The fact is that neither contractile nor enzymatic action of rHuEpo was accompanied with the improvement of the functional renal and haematologic parameters, indicating a primary effect on myocardial contractile function of rHuEpo, independent of the anaemic and uraemic state of the animal. The reason why erythropoietin is able to modulate directly the cardiac Na<sup>+</sup>/K<sup>+</sup> pump makes it possible to conclude that the lack of erythropoietin in CRF may be at least in part responsible for the inhibition of cardiac enzymes, altering the contractile behaviour of the heart.