The effects of DNP on the electrical activity of single cells of the sinus node, right auricle and A-V node, were studied with microelectrodes. It was observed that this agent caused an initial tachycardia followed by a bradycardia, a decrease in the duration of the action potential, a decrease of the resting potential and a decrease in the slope of diastolic depolarization of pacemaker tissues. A complete inhibition of the electrical activity was observed in 45 minutes after the addition of DNP to the perfusion fluid. A similar result was obtained with sodium azide. A significant recovery of the electrical activity was obtained with the use of a system with DNP and ATP. A possible increase of the extracellular potassium concentration produced by DNP similar to that observed in skeletal muscle is discussed. An incomplete loss of the resting potential observed with DNP makes it probable that at least a fraction of the resting potential is independent of the energy supplied by oxidative phosphorylation.