Inward currents of the rabbit sinoatrial node cell were examined in voltage-clamp experiments using the two-microelectrode technique. A fast and slow inward current could be separated from each other. The slow inward current was blocked by Mn and D 600, but it was insensitive to TTX. On the contrary, the fast inward current was blocked by TTX, but not by Mn and D 600. Both the fast and slow inward currents disappeared on Na removal. The fast inward current system was fully inactivated by holding the membrane potential positive to -40 to -50 mV, while the slow inward current system was recorded with the holding potential up to -20 mV. The voltage dependence of the inactivation of the 2 inward current systems and their dependence on [Na]0 suggests that the rising phase of the spontaneous action potential in the S-A node cell is produced mainly by Na current through the slow inward current system.