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Introduction: In resent years, neuronal type Na+ channel is one of the important currents for action potential depolarization phase in heart cells. In this study neuronal type Na+ channel is blocked by low concentration of Tetrodotoxin (TTX) to compare the effect of TTX blocker on pacemaker activity of sinoatrial node (SAN) and atrioventricular node (AVN) of mouse heart. Materials and Methods: In this experimental study the pacemaker activity of distinct intact SAN and AVN, was recorded before and during consuming 100 nM TTX, and cycle length (CL) was measured. Data was analyzed using t-test. Results: 100 nM TTX increased CL on SAN preparations by 22.2±6% and on AVN preparations by 52.5±13.5 %. These changes were significant in the two nodes. Conclusion: It is passible to conclude that the neuronal type Na+ channel was present in the two nodes, and the effect of TTX on CL of the two nodes was different.