Experiments were performed on isolated rat soleus (slow-twitch) and extensor digitorum longus (EDL) (fast-twitch) muscle of 4-week-old rats. In soleus muscle, electrical simulation at 2 Hz for 5 min increased the ouabain-suppressible 86Rb+uptake by 138%, without significant changes in intracellular Na+content or Na+/K+ratio. In EDL muscle, the ouabain-suppressible 86Rb+uptake was stimulated by only 58%, whereas intracellular Na+content and Na+/K+ratio were increased by around 70%. Na+-loading of the muscles by exposure to K+-free or K+-Ca2-Mg2+-free buffer stimulated the ouabain-suppressible 86Rb+uptake in the two muscles to roughly the same extent, but in EDL muscle this was associated with a more than twofold larger increase in Na+/K+ratio. When the Na+influx was increased by exposure to veratridine similar results were obtained. Graded variation in intracellular Na+content was achieved by exposure to monensin. In soleus muscle, a 25% increase in intracellular Na+/K+ratio resulted in a doubling of the ouabain-suppressible 86Rb+uptake, whereas a doubling of the Na+–K+transport rate in EDL muscle required a 140% increase in Na+/K+ratio. The results indicate that in soleus muscle the Na+/K+pump is much more sensitive to changes in intracellular Na+content than in EDL muscle. This might explain the larger activation of the Na+–K+pump in slow-twitch muscle during electrical stimulation and might be of significance for the activation of the Na+-K+pump in vivo during work.