Recording cardiac electrical activity after a countershock has been limited by amplifier saturation. Modifications to our computer-assisted mapping system allowed us to record electrical activity from 56 epicardial electrodes within 5 ms of the end of a countershock. Modifications included the use of solid-state switches to disconnect the filter section of the amplifiers during the shock and changing the low-frequency response to the amplifiers from 0.1 to 10 Hz to filter out large, low-frequency potentials after the shock. Six-millisecond truncated exponential shocks were delivered between the superior vena cava and right ventricular apex through a quadripolar catheter during normal rhythm in seven dogs. As shocks of increasing voltage were delivered during the T-Q interval, progressively more of the epicardium was directly depolarized. A shock of 109 ± 17 (SD) V directly depolarized the entire epicardium. Shocks of constant voltage were then delivered with increasing prematurity during diastole. As the ventricles became more refractory with increasing shock prematurity, the amount of epicardium depolarized became progressively less. Thus computer-assisted mapping techniques are capable of measuring the area depolarized by a shock during normal rhythm and may be useful during arrhythmias to improve our understanding of defibrillation and cardioversion.