Abstract: In whole cell voltage clamp experiment with isolated guinea pig ventricularmyocytes,tocainide was shown to exhibit a concentration-dependent inhibition of calcium current(I_ca)and deldyed rectifiet potassium current(I_k).Tocainide with a therapeutic concentration of 50umol·L^-1 significantly inhibited the I_ca and Itail(deactivating tail current of Ik)by 16%and 3%respectively.Inhibition of I_ca by tocainide was also shown to be dependent upon stimulatingfrequencies,with a greater blockade occurring at 2.0 Hz (57%)than at 0.2 Hz(17%).Tocainidewas found to inhibit the I_k,ATP by 74%at 200 umol·L^-1 but not at 50 umol·L^-1.These inhibitoryeffects on Ica and I_k can probably explain the therapeutic effect of tocainide on supraventriculartachycardia,and the shortening of plateau phase of action potential.