Recent electrophysiological data have provided the evidences that background currents such as Na(+)-Ca²⁺ exchange can significantly modulate cardiac pacemaker activity. In this study, the effects of extracellular Na⁺ and Ca²⁺ concentrations on the pacemaker activity were investigated by measuring the intracellular Na⁺ activity (aⁱ_Na) with Na⁺-selective microelectrodes and the results are summarized as follows. 1) In the rabbit SA node, aⁱ_Na was 3.2 ± 0.3 mM and mean MDP (maximal diastolic potential) was -63.3 ± 1.4 mV. 2) Graded decreases of external Na⁺ concentration resulted in the loss of spontaneous beating, hyperpolarization and the decrease of aⁱ_Na. 3) An increase in extracellular Ca²⁺ concentration in low Na⁺ solution augmented the transient decrease of aⁱ_Na, about 3 minutes in low Na⁺ solution, until aⁱ_Na started to increase. 4) In low Na⁺ solution, which had extracellular Ca²⁺ concentration according to the calculation based on the equilibrium state of Na⁺-Ca²⁺ exchange, aⁱ_Na was continuously decreased. It was concluded that intracellular Na⁺ activity modulated by Na⁺-Ca²⁺ exchange could play an important role in the initiation of pacemaker potential.