We used cell pairs electrically coupled with relatively high intercellular resistance to investigate the involvement of calcium current in the origin of the source current during the conduction process of the action potential (AP). Three interventions were used to reduce the calcium current: a specific calcium channel blocker [nifedipine (NIF)], premature stimulation, and increments in the frequency of stimulation of the cell. The ionic membrane current (Iion) after the peak of the AP of the stimulated cell was positive and small when the cell was uncoupled. However, when the stimulated cell was coupled to a cell model or to another cell, Iion during this period became negative and large to supply the coupling current. A rapid early repolarization of the AP occurred in the stimulated cell because of the removal of charge from the stimulated cell. NIF decreased the magnitude of the net negative Iion during this period and caused a more rapid early repolarization in the stimulated cell. NIF increased the delay between the activations of two coupled cells at a given coupling resistance (Rc) but decreased the longest delay that could be produced without conduction failure for a given cell pair. The highest Rc below which conduction of AP occurred was also decreased by NIF. Premature stimulation and an increase of the stimulation frequency also caused an increase in the extent of the early repolarization and increased the delay between two cell activations at a given Rc. Conduction block occurred with sufficient prematurity or at a sufficiently high frequency of stimulation even though activation of the stimulated cell occurred for each stimulus. The Iion that flows during the early plateau phase of the AP in the stimulated cell became negative and significantly large by coupling two cardiac cells together. This current flow is a major component needed to supply the coupling current through the intercellular resistance. The decrease of calcium current caused a decrease in the magnitude of this net inward ionic current, resulting in an increase of the rate of early repolarization and an increase in the conduction delay between two cells at a given Rc. These results suggest the involvement of calcium current in the conduction process when cells are coupled at relatively high Rc.