Canine cardiac Purkinje fibers exposed to sodium-free solutions containing 16 mM CaCl2, 20 mM tetraethylammonium chloride, 108 mM tetramethylammonium chloride, and 2.7 mM KCl may be quiescent at a resting potential of either -50 mV or -90 mV. The membrane potential of these fibers can be switched from -50 mV to -90 mV by a hyperpolarizing current pulse and from -90 mV to -50 mV by a depolarizing current pulse. The transition from -50 mV to -90 mV depends on a voltage-dependent increase in potassium conductance, that conductance being low at -50 mV and high at -90 mV. A reduction in potassium conductance causes the fiber to depolarize from -90 mV to -50 mV because of the presence of an inward current which apparently is carried mainly by Ca. Fibers that show a high resting potential cannot be excited except by depolarizing stimuli strong enough to move the membrane from -90 mV to a threshold potential of about -40 mV. Fibers that show a low resting potential are more easily excited and may show rhythmic activity sustained by afterpotentials that appear only if the low membrane potential is accompanied by a low potassium conductance. Slow changes in membrane potential also are seen; these changes may result from movements of chloride.