The resting membrane potential and intracellular potassium and sodium concentration of three guinea pig hind limb muscles were studied. These properties are related to the gross color, the speed of contraction, and the biochemically defined fiber type composing the muscle. The resting membrane potential and intracellular content were: white vastus (grossly white, fast-twitch glycolytic) -85.3 mV. potassium 171.9 meq/liter; soleus (grossly red, slow-twitch oxidative) -69.7 mV, potassium 137.5 meq/liter; and red vastus lateralis (grossly red, fast-twitch oxidative glycolytic) -71.7 mV, potassium 139.6 meq/liter. In soleus and red vastus lateralis, the relative permeability of sodium to potassium was 0.041 and 0.036, while in white vastus it was 0.015. These results give us the first exception to the hypothesis that fast-twitch fibers have higher intracellular potassium and higher resting membrane potential than slow-twitch fibers.