1. Intact skeletal muscle fibres have been shown to contain a high concentration of [3H]ouabain binding sites (100-800 pmol g wet wt.-1). Under resting conditions, however, it seems that in isolated muscles only 2-6% of the corresponding expected capacity for active Na+-K+ transport is utilized. 2. In order to determine whether all [3H]ouabain binding sites in rat soleus muscle represent functional Na+-K+ pumps, we have measured the maximum rates of the ouabain-suppressible components of isotopic fluxes of Na+ and K+ as well as the net changes in Na+-K+ contents. 3. Experiments with soleus muscles isolated from 4-week-old rats showed that following Na+ loading (I.C. Na+, 126 mmol l-1), the ouabain-suppressible 86Rb+ uptake and 22Na+ efflux as measured during 3 min of exposure to K+-rich buffer were 5800 and 6500 nmol g wet wt.-1 min-1, respectively. 4. These initial high rates of isotopic fluxes were confirmed by flame photometric measurements of Na+-K+ contents. The ouabain-suppressible 86Rb+ uptake had a temperature coefficient of 2.1, was inhibited by 2,4-dinitrophenol, but showed no response to tetracaine, BaCl2, Ca2+-free buffer or tetraethylammonium chloride. 5. In soleus muscles, where the total population of [3H]ouabain binding sites had undergone changes as a result of differentiation, K+ depletion or pre-treatment with thyroid hormone, there was a significant correlation (r = 0.95, P less than 0.005) between the concentration of [3H]ouabain binding sites (260-1170 pmol g wet wt.-1) and the maximum ouabain-suppressible 86Rb+ uptake (2300-10,900 nmol g wet wt.-1 min-1). 6. It is concluded that by the combination of Na+ loading and high extracellular K+, the available Na+-K+ pumps as quantified by the [3H]ouabain binding capacity can be activated to reach a transport rate around 90% of the theoretical maximum at 30 degrees C.