Abstract: The effects of nicotinamide on isolated rabbit S-A node were studied with standard microelectrode techniques. At a concentration of 4 mg/ml of nicotinamide, the slope of phase 4 of the S-A node cell action potential was significantly increased, the spontaneous sinus cycle length and action potential duration at 90% repolarization were markedly shortened. The rate of depolarization of phase O and the action potential amplitude were unchanged. These effects of nicotinamide were similar to those of isoproterenol (at a concentration of 0.03 mg/L). The effect of nicotinamide could be blocked by verapamil at a concentration of 0.1 mg/L which has no significant effect on the action of isoproterenol. Propranolol at a concentration of 0.25 rag/L, which blocked the effect of isoproterenol completely, showed no effect on the action of nicotinamide. In the experiment, nicotinamide can prevent the slope of phase 4 from being decreased and the sinus spontaneous cycle length from being prolonged by the action of verapamil, but the action of verapamil to decrease the slope of phase O and the amplitude of action potential was not affected. The results suggest that nicotinamide might be an activator of Ca⁽⁺⁺⁾ channel. Nicotinamide could promote the Ca⁺⁺ inward current of phase 4.