The Na,K-ATPase is of major importance for active ion transport across the sarcolemma and thus for electrical as well as contractile function of the myocardium. Furthermore, it is receptor for digitalis glycosides. In human studies of the regulatory aspects of myocardial Na,K-ATPase concentration a major problem has been to obtain tissue samples. Methodological accomplishments in quantification of myocardial Na,K-ATPase using vanadate facilitated 3H-ouabain binding to intact samples have, however, made it possible to obtain reliable measurements on human myocardial necropsies obtained at autopsy as well as on biopsies of a wet weight of only 1-2 mg obtained during heart catheterisation. However, access to the ultimately, normal, vital myocardial tissue has come from the heart transplantation programs, through which myocardial samples from cardiovascular healthy organ donors have become available. In the present paper we evaluate the various values reported for normal human myocardial Na,K-ATPase concentration, its regulation in heart disease and the association with digitalization. Normal myocardial Na,K-ATPase concentration level is found to be 700 pmol/g wet weight. No major variations were found between or within the walls of the heart ventricles. During the first few years of life a marked decrease in myocardial Na,K-ATPase concentration is followed by a stable level obtained in early adulthood and normally maintained throughout life. In patients with enlarged cardiac x-ray silhouette a significant positive, linear correlation between left ventricular ejection fraction (EF) and Na,K-ATPase concentration was established. A maximum reduction in Na,K-ATPase concentration of 89% was obtained when EF was reduced to 20%. Generally, heart failure associated with heart dilatation, myocardial hypertrophy as well as ischaemic heart disease is associated with reductions in myocardial Na,K-ATPase concentration of around 25%. During digoxin treatment of heart failure patients a further reduction in functional myocardial Na,K-ATPase concentration of 15% has been found. Thus, the total reduction in functional myocardial Na,K-ATPase concentration in digitalised heart failure patients may well be of the magnitude 40%. In conclusion, it has become possible to quantify human myocardial Na,K-ATPase in health and disease. Revealed reductions are in heart failure of importance for contractile function, generation of arrhythmia and for digoxin treatment.