The effect of repeated countershocks on transthoracic apparent impedance to direct current (dc) defibrillator discharges was studied. Repeated dc countershocks result in a progressive decrease in transthoracic apparent impedance that is dependent upon the time interval between countershocks. This decrease was significantly greater in the group of animals shocked at 3-min intervals compared to the groups shocked at 15-sec intervals (P less than 0.001) or at 1-min intervals (P less than 0.005). Since lowered impedance results in higher delivered current for the same energy setting on a defibrillator, this observation may help to explain the enhanced effectiveness of repeated countershocks in defibrillation. Plots of simultaneous current against voltage during transthoracic dc discharge revealed that the current lagged slightly behind voltage during the rising phase of the recording, but that current and voltage were nearly simultaneous during the falling phase. This effect appears to be similar to an ionization phenomenon in that the effective impedance asymptotically approaches a lower value with increasingly applied voltage. This might explain why transthoracic impedance is highest at low energy countershocks and decreases with higher energy countershock.