An algorithm is developed to simulate defibrillation (DF)-level electric shocks on a 3D computer model, HeartSim, of the heart ventricles. These defibrillation-level shocks are based on current density distributions calculated from finite element simulations. Successful DF shocks produce a long period of inactivation (>200 ms) when almost no activation occurs in the heart. In contrast, unsuccessful pulses produce a short period (ave. = 53.8) of low activation after which reentrant loops are initiated by the nondepolarized cells. As a result of single pulse DF simulations on HeartSim, it is found that the success of a DF shock is dependent on the number of elements excited by the shock, the relative positions of these elements at the time of the shock, and the extension of the refractory period of the elements. In addition to single DF pulses, two square pulses are simulated, each half the duration of a single pulse, and separated by a 65-msec interval. This results in a 68% reduction in DF threshold energy from that required by a single DF pulse.