The effects of reduced Na+,K+ pump capacity on contractile endurance and excitation-induced changes in intracellular Na+ content were investigated in isolated rat soleus and extensor digitorum longus muscles. Pre-incubation with 10-5m ouabain increased the rate of force decline measured over the first 5–20 s of tetanic contraction from 0.32 to 0.94% s-1 and 1.4 to 4.6% s-1 in soleus and extensor digitorum longus muscles, respectively. Soleus muscles from K+-deficient rats exhibited 54% reduction in the concentration of Na+,K+ pumps and the force decline during 30 s of 60 Hz stimulation was increased from 0.53 to 1.15% s-1. A similar change was induced in control muscles when a comparable reduction in the concentration of functional Na+,K+ pumps was elicited by pre-incubation with ouabain (10-6-2×10-6m). In soleus, the force decline during 60 s of 60 Hz stimulation showed linear correlation to the increase in intracellular Na+ content. In extensor digitorum longus, force decline and increase in Na+ content during 60 Hz stimulation were both four times faster than in soleus as measured over 15 s of excitation. These results indicate that during maximal contractions the Na+,K+ pump capacity is one of the determinants for the contractile endurance in skeletal muscle. Furthermore, the maintenance of contractile force seems to be a function of the rate of Na+-influx and this relationship may account for the difference in endurance between slow-twitch and fast-twitch muscles.