Cardiac pacemaking is generated by the action potential from sinoatrial node which is consist with various membrane currents. In this study, we investigated the influence of membrane currents on the sinoatrial node automaticity. The action potential of sinoatrial node were measured with micro electrode method in guinea pig isolated right atrial tissue preparation. The beating rate of sinoatrial node was decreased by both 1 μM E-4031, a selective IKr blocker, and 30 μM Chromanol 293B, a selective IKs blocker. E-4031 prolonged APD50 and APD90, while Chromanol 293B prolonged APD20, APD50 and APD90. The maximum diastolic potential was increased by E-4031 but not by Chromanol 293B. These results suggest that IKs and IKr have different roles in guinea pig sinoatrial node automaticity; IKr may contribute to the late repolarization phase and diastolic depolarization, whereas IKs may contribute to the early repolarization phase.