To estimate the "resting potential" of the sinoatrial node cell, membrane potentials in quiescent states under various conditions were compared with the holding potential at which no net membrane current flowed during the voltage clamp. The membrane potential of the rabbit sinoatrial node cell was recorded using a conventional microelectrode method. The single sucrose gap method and the double microelectrode method were employed for the voltage clamp. The membrane potential immediately before the resumption of the spontaneous activity in normal Tyrode solution after a quiescence in high K+ concentration, in low Na+ concentration or in low temperature was approximately - 37 m V and was similar to that of the temporarily quiescent sinoatrial node cells after dissection. The holding potential at which no net membrane current flowed was - 38.4 m V, which coincided with the membrane potential during quiescence. The resting potential in quiescence increased by approximately 17 m V for a tenfold decrease in the extracellular Na+ concentration, and 22 m V for a tenfold increase in the K+ concentration. The relative membrane conductance decreased transiently on removing Na+ from the bathing medium. These findings suggest that a potential equivalent to the resting potential existss in the S-A node cell approximately 20 m V positive to the maximum diastolic potential, and this low value might be due to the high Na+ conductance of the cell membrane.