The relationship between the intracellular cation concentration and the membrane potential has been studied in "Na-rich" soleus muscle fibers of rats which had been fed a K-free diet for 10-50 days. The resting potentials of "Na-rich" muscle fibers closely agreed with the theoretical potential expected from ionic theory when a quantitative dissociation of active cation transport with Na ions extrusion exceeding K ions uptake was eliminated due to the recovery of "Na-rich" fibers in Krebs solution with 10 mM K for 2 h at 37 degrees C. The hyperpolarized membrane potentials during cellular Na ions extrusion were accounted for by the sum of the potentials produced by the electrogenic Na-pump and by the ionic diffusion potential. On the other hand, the amplitude of overshoot of action potentials decreased linearly with the logarithmic increase of the intracellular Na concentration ([Na]i). The maximum rate of rise of action potentials also changed as a fraction of [Na]i, though, at the early period of K-deficiency the inhibitory effect of the increased [Na]i on the maximum rate of rise was transiently masked by the hyperpolarization produced by the electrogenic Na-pump which secondarily led to a progressive reduction of Na inactivation, while the maximum rate of fall was a linear function of [K]i.