Abstract: The mechanism of antiarrhythmic action and the electrophysiologic effects of 7-bromoethoxybenzene-tetrahydropatmatine (EBP)have been studied using conventional microelectrode technique. The effect of EBP on the membrane Ⅰ_Si and ⅠX currents were investigated in the canine cardiac purkinje fibres using the double microelectrode voltage clamp methods. EBP was shown to increase the duration of action potential at 20 and 90% of repolarization of isolated guinea pig papillary muscles. However, the amplitude of action potential, the resting potential and the overshoot, and maximum rate of 0 phase depolarization (Ⅴ_max) remained unchanged. The membrane potential was held at 40 mV and command potential at-15mV, 0.3 Hz 500 ms. After,10 min of perfusion of EBP 30 μmol/L, the slow inward current (Ⅰ_Si)was reduced markedly. When the holding potential Was held at-20 mV., and the command potential at+10 mV, EBP (10～100μmol/L)exerted a depressed effect on the delayed (outward) rectifier current (Ⅰ_X) in a dose-dependent manner. It may be concluded that the antiarrhythmic effects of EBP is mainly related to decrease the automaticity and prolong the duration of action potential at 20～90% of repolarization of isolated guinea pig papillary muscles.