Resumen: We develop and validate a three-dimensional model of ventricular fibrillation for use in studying defibrillation. Specifically, we use a rabbit ventricular geometry as a realistic model system for evaluating the efficacy of defibrillatory shocks. Data obtained from the simulations are analyzed in terms of a dose-response curve. In agreement with clinically relevant values, we find that an electric field strength of about 6.6 V/cm indicates a 50% probability of successful defibrillation for a 12-ms monophasic shock in our model. Our validated model will be useful for optimizing defibrillation protocols and for performing detailed studies of defibrillation mechanisms.