In hypokalaemic rats maintained on a potassium deficient diets for 10-50 days, the isolated "Na-loaded" and "K-depleted" ("Na-rich") muscle fibers showed the membrane potential less than -115 mV in "fresh" muscles of normal rats in K+-free Krebs solution. Upon adding 5 mM K+ to the K+-free medium bathing the soleus muscles, the measured potentials of "Na-rich" muscles always exceeded the membrane potentials of "fresh" muscles in 5 mM K+. The hyperpolarization was dependent on the amount of intracellular Na+ concentration (["na]i) accumulated during the potassium deficiency. The electrogenic Na-pump was activated by an increase of [Na]i of less than 5 mM. Further increases in [Na]i resulted in increases in membrane potential which appeared to approach a limit at [Na]i levels higher than 65 mM.