Abstract: Effects of rhynchophylline(Rhy)on the voltage- dependent potassium channels inisolated rat and guinea pig ventricular myocytes were studied by using patch clamp whole cell recordingtechniques.Rhy was found to reduce the steady- state outward current in rat ventricular myocyteswhen cells were depolarized from holding potential一40mV to 0 mV,After blocking Ca²⁺ inwardcurrent with CdCl₂ 0.1 mmol/L,a K⁺transient outward current(I_A)can be recorded. Rhy 30μmol/L inhibited the I_A 30% when depolarized to+50 mV from the holding potential.The delayedoutward(I_K)and outward tail current(I_tail)were also reduced 30%and 60%, respectively by Rhy30μmol/Lwhich responsed to 4.5 sec long pulse from一40mV to+50 mV. Whereas,the inwardrectifying current(I_Kl) was not changed by Rhy. The present study suggests that the antiarrhythmicaction of Rhy was partially due to the K+channel blocking effects.