Na,K-ATPase (or the Na,K-pump) is essential for excitability and contractility of muscle tissue. Previous studies have shown a decrease in the concentration of this pump in endomyocardial biopsies from patients with dilated cardiomyopathy. The effect of congestive heart failure on the concentration of Na,K-ATPase in skeletal muscle was assessed in 16 patients by measurement of binding of 3H-ouabain to biopsies of the vastus lateralis muscle. Ten patients had impaired left ventricular function with an ejection fraction of 0.32 ± 0.03 and a concentration of the Na,K-pump of 229 ± 15 pmol/g wet weight in the skeletal muscle, whereas 6 patients had an ejection fraction of 0.66 ± 0.05 (P < 0.001) and a concentration of 307 ± 17 pmol/g wet weight (P < 0.01). In endomyocardial biopsies, the concentration of Na,K-ATPase was 340 ± 37 and 500 ± 39 pmol/g wet weight (P < 0.025) in patients with impaired and normal ventricular function, respectively. There was a significant correlation between the concentration of the Na,K-pump in the biopsies of the skeletal muscle and ejection fraction, as well as between its concentration in the endomyocardial and skeletal muscular biopsies (r = 0.56, P < 0.025 and r = 0.72, P < 0.005, respectively). The decrease in concentration of the pump in skeletal muscle may contribute to the limitation of exercise capacity in congestive heart failure.