The maximum rate of rise of depolarization of the isolated rabbit S-A node cells was studied with an intracellular microelectrode. Both the amplitude and the maximum rate of rise of depolarization reduced in proportion to the extracellular sodium concentration, but the reduction in the maximum rate of rise of depolarization was greater than that in the amplitude of action potentials. The frequency of spontaneous activities and the duration of action potentials were increased during application of depolarizing current, while they were both reduced during application of hyperpolarizing current. Both the amplitude of the action potential and the maximum rate of rise of depolarization showed voltage dependent characteristics. The maximum value of the maximum rate of rise of depolarization in pacemaking cells ranged from 2 to 11 V/sec and was quantitatively lower than other myocardial fibers. The maximum value of the maximum rate of rise of depolarization in 50% Na+ solution was reduced to approximately 1/2 of the control in normal Na+ solution. A signoidal relationship was observed between the membrane potential and the maximum rate of rise of depolarization. When the temperature of the perfusate was lowered, the signoidal curve shifted toward the negative direction. It is concluded that sodium ion is responsible for the generation of the initial phase of the S-A node action potentials, as in other myocardial cells. However, the amount of ionic current required for spike generation is quantitatively smaller than other myocardial cells.