The excitation threshold of isolated cardiac cells has been shown to be sensitive to the direction of applied electric field. To further explore this relationship, we developed a realistic two-dimensional finite element model of a cardiac cell. The model was used to determine the spatial distributions of transmembrane voltages produced by a uniform electric field applied across the cell. With a 5 V/cm field applied parallel to the cell axis, the maximum absolute transmembrane voltages measured at either end of the cell were 39.1 mV and 46.5 mV (signs depend on polarity of applied field), while 40.5 mV and 44.8 mV with the field perpendicular to the cell axis. More significantly however, we found that these highest potentials were concentrated at distinct sites on the membrane. Thus, we hypothesize that the depolarization of a cell due to the defibrillation shock initiates at one of these 'hot spots' whose exact location depends on the direction and polarity of the field, and shape of the cell. Our computational results are in good agreement with experimental results suggesting that a nonuniform cell shape does have an important bearing on the subsequent excitation thresholds of that cell.