Abstract: AIM　To investigate the effects of benzyltetrahydropalmatine (BTHP) on the action potential (AP) of guinea pig papillary muscle and delayed rectifier potassium currents (I_K )of single ventricular myocyte when administered intracellularly. METHODS　Standard microelectrode was applied to examine the characteristics of the AP. BTHP included in the pipette internal solution was dialyzed into papillary muscle cell by using a pulse with duration of 50 ms, voltage of +40 V. The parameters of AP were recorded after 20 min of intracellular dialysis. Whole cell patch clamp technique was used to measure the I_K and BTHP was applied intracellularly in single ventricular myocyte by cell dialysis method. RESULTS　100 μmol.L^-1 BTHP prolonged APD20 and APD90 by 13.5% and 20.5%, respectively. Furthermore, 30　μmol.L^-1 BTHP was demonstrated to decline I_K and I_K,tail from (14.1±2.2) pA.pF^-1 and (4.0±0.6) pA.pF^-1 to (9.4±1.3) pA.pF^-1 and (2.6±1.0) pA.pF^-1 with inhibition rates of 33.2% and 35.3%, respectively. BTHP reduced the amplitude of I-V curves of I_K and IK,tail, but did not affect the shape of them significantly. CONCLUSION The results showed that BTHP used intracellularly can block IK and prolong the duration of action potential(APD).