To investigate whether the sodium-potassium pump or Na+,K+-ATPase concentration in muscles is related to the capacity for muscle performance, the 3H-ouabain binding site concentration or 3-O-methylflourescein phosphatase activity was determined in samples of heart ventricular muscles from trained rats, cardiomyopathic hamsters, and human subjects. These methods have earlier been shown to quantify the Na+,K+-ATPase concentration in muscle tissue with high accuracy. When rats were swim trained for six weeks the heart ventricular muscle Na+,K+-ATPase concentration was increased by 20% (p less than 0.02) and the heart to body weight ratio by 14% (p less than 0.005). The increase in Na+,K+-ATPase concentration was only slowly reversible. After three weeks of deconditioning an increase of 12% (p less than 0.05) was still observed. In comparison skeletal muscle Na+,K+-ATPase concentration was increased by up to 46% (p less than 0.001) and decreased by up to 30% (p less than 0.005) after training and immobilisation respectively. Cardiomyopathic hamsters showed a reduction of 33% (p less than 0.005) in the heart ventricular Na+,K+-ATPase concentration compared with normal hamsters. This decrease was associated with a heart to body weight increase of 19% (p less than 0.01) and congestive heart failure. In six patients with a mean ventricular ejection fraction of 68% the mean Na+,K+-ATPase concentration in endomyocardial biopsy specimens obtained by left heart catheterisation was 505(41) compared with 322(32) pmol.g-1 wet wt (p less than 0.001) in nine patients with a mean ejection fraction of 29%. Taken together the present data indicate a relation between the Na+,K+-ATPase concentration and the capacity for muscle performance.