The authors present a three-dimensional computer model of the heart to demonstrate simulation of ventricular fibrillation (VF) and defibrillation (DF). Delivery of a run of extra-stimuli initiated self-sustaining rapid regular rhythm or coarse and fine VF. Shocks of increasing strengths were applied at various times to determine the energy required for successful DF. The success rate curve versus DF energy showed the expected sigmoidal shape. For a given episode, the energy required for DF showed remarkable variation from moment to moment. Two successive pulses significantly lowered the energy requirement. The optimum interval between pulses was found to center around 85 ms, and the energy for the second pulse was lower than that for the first pulse. A biphasic pulse also lowered the energy requirement.