The effects of denervation and ouabain on the relationship between resting membrane potential (EM) and extracellular potassium concentration were determined for rat soleus (SOL) and extensor digitorum longus (EDL) muscles. At concentrations above 20 mM for both SOL AND EDL fibres the relationship could be described adequately by the Nernst equation. Denervation resulted in reduction of EM and a decrease in the slope of the relationship between EM and potassium concentration. 10(-3) M ouabain produced the same effects as denervation. The results are discussed in relation to the alterations in membrane permeability previously shown to occur as a consequence of denervation. It is concluded that the low EM of denervated muscle is the result of an increase in membrane permeability to sodium.