The relationship between the number of 3H-ouabain binding sites and the Na, K-pump mediated K-uptake has been characterized in rat soleus muscle. By brief exposure to 3H-ouabain (1 X 10(-6)-1 X 10(-5) mol/l) in vitro, it could be measured that 19-94% of the ouabain binding sites had been occupied. This was associated with a proportionate decrease in the ouabain suppressible K-uptake indicating that under strictly standardized conditions, measurements of 3H-ouabain binding sites quantify functional Na,K-pumps. When 3 week old rats were K-depleted for a further week followed by K-repletion 2 h before measurements, the 3H-ouabain binding site concentration was 61% lower than in age-matched control soleus muscles. However, the ouabain suppressible K-uptake was only reduced by 35%, partly because intracellular Na remained higher in the muscles obtained from K-depleted rats. From the 1st to the 4th week of life, the 3H-ouabain binding site concentration increased 2.9-fold. In contrast, the ouabain suppressible K-uptake decreased by a factor 3.5. Accordingly, in muscles from 1 week old rats, the ouabain suppressible K-uptake per 3H-ouabain binding site was 10-fold higher than in muscles from 4 week old rats. This difference could not be accounted for by changes in intracellular Na, total or extracellular water. It may be related to differentiation and change in structure.(ABSTRACT TRUNCATED AT 250 WORDS)