Abstract: Guan-fu base A (GFA) is a terpenoid alkaloid isolated from the tuber of Aconitum coreanum, which has been shown to prolong cardiac repolarization in vivo and in vitro . In the present study, the effects of GFA on the delayed rectifier current(I_k) were investigated using the whole cell patch clamp technique in isolated guinea pig ventricular myocytes. In the presence of CdCl₂ 100 μmol·L^-1 , I_k was observed upon depolarizing pulses to +50 mV from a holding potential -40 mV for variable duration (550, 1100, 1650, 2200, 2750, 3300, 3850 ms). The magnitude of I_k after a 2200 ms pulse was 293±90 pA prior to drug and 227±59 pA in the presence of GFA 100 μmol·L^-1 . In a 3850 ms pulse, the magnitude of I_k decreased from 290±90 to 231±66 pA after exposure to GFA. The inhibitory effects of GFA on I_k was not dependent on the duration of depolarization. The inward rectifier current (I_k1) was not affected by GFA 100 μmol·L^-1 . It is concluded that GFA has inhibitory effects on I_k, which may contribute to its prolongation of cardiac repolarization.