Abstract: AIM: To observe the cellular electrophysiological effect of 3,6-dimethylamino-dibenzopyriodonium of ferric EDTA(IHC-66, 1～50 μmol．L^-1). METHODS: Using myocardium intracellular microelectrode techniques in isolated canine cardiac Purkinje fibers and ventricular muscles. RESULTS: The initial effects of IHC-66 consisted of shortening of the duration of action potential measured at 20%(APD₂₀) and 50%(APD₅₀) of repolarization while the maximal depolarization rate of action potential of phase 0 (Vmax) was decreased in isolated ventricular muscles and APD₅₀ was shortened in isolated canine cardiac Purkinje fibers when the preparation was superfused with bath solution containing IHC-66 5 μmol．L^-1 for 15 min. The inhibition effects of APD₂₀ and Vmax were also observed in Purkinje fibers with increasing drug concentration (10 and 30 μmol．L^-1). At higher concentration (30 μmol．L^-1 and above), IHC-66 decreased the amplitude of action potential and prolonged the duration of action potential measured at 90%(APD₉₀) of repolarization in isolated ventricular muscles of canine cardiac Purkinje fibers but without any significant effect on the resting potential. IHC-66 30 μmol．L^-1 produced a frequency-dependent depression of Vmax while altering stimulating frequency between 0.3～5 Hz in isolated Purkinje fibers. CONCLUSION: The inhibition effects of IHC-66 on action potentials in isolated canine cardiac muscles were more powerful than that in isolated canine cardiac Purkinje fibers. IHC-66(30 μmol．L^-1) produced a frequency- dependent depression of Vmax.