AIM: In order to further study on electrophysiological properties of IHC 66 3,6 di(dimethylamino) dibenzopyriodonium of ferric EDTA and to evaluate its mechanism. METHODS: the effects of IHC 66 on action potential (AP) of guinea pig sinoatrial node (SAN) cell, papillary muscle cell depolarized by high K + and abnormal automaticity induced by Ba + (2 mmol·L -1 ) in papillary muscle cell were examined using the conventional microelectrode technique. RESULTS: IHC 66 10 μmol·L -1 decreased the slope of phase 4 (SP 4) of AP of SAN cell by 11 42%; increased 90% action potential duration (APD 90 ) of SAN cell by 18%; decreased the maximum rate of rise of the action potential (V max ) of SAN cell by 47%. APD 90 、50% action potential duration (APD 50 ) of depolarized papillary muscle cell was shortened by 9%、13%, respectively, the V max of depolarized papillary muscle cell was decreased by 58%. IHC 66 (10、30、50 μmol·L -1 ) dose dependently decreased action potential amplitude(APA) and V max of abnormal automaticity induced by barium in papillary muscle cell; shortened the APD 50 、APD 90 of such an abnormal automaticity and decreased spontaneous firing frequency apparently. CONCLUSION: These results suggest that the cellular electrophysiological effects produced by IHC 66 in isolated guinea pig cardic preparations are mainly concerned with its inhibition on slow influx current.