Background. Defibrillation may be improved if electrode configurations can be found that create a larger and more even voltage gradient field across the heart. This study determined the magnitude of the shock gradient fields generated by four nonthoracotomy electrode configurations for defibrillation. Methods and Results. In six dogs, a catheter was inserted containing a right ventricular apical electrode (V) and a right atrial electrode (A). A cutaneous patch electrode (P) was placed on the left lateral thorax. Shock potentials were recorded simultaneously from 128 electrodes in the left ventricular and right ventricular subepicardium and subendocardium, ventricular septum, and atria. With the chest closed, 50-mA shocks were given during diastole via the following lead configurations: V→A (V, cathode; A, anode); V→P; V→A+P; and V+A→P. Potential gradients were calculated at the subepicardium and subendocardium in millivolts per centimeter per volt of shock. In most dogs, the V→A+P configuration produced higher gradients throughout the ventricles than did V→A, V→P, or V+A→P. The maximum potential gradient was smaller for the V+A→P configuration than for V→A, V→P, or V→A+P. The gradient fields for the configurations with the catheter alone or combined with P were uneven. Conclusions. It is possible to estimate shock gradient fields in three dimensions. Of the four configurations tested, V→A+P produced the highest gradients and V+A→P produced the lowest high gradient. The gradient fields were uneven throughout the ventricles.