The purpose of this study was to characterize the electrophysiological effects of suprathreshold transthoracic shocks. Dogs were anesthetized and two consecutive shocks (10 joules/gk) were applied one minute apart. Ventricular extrasystoles were observed in six of the 11 dogs tested, A-V block in three, ventricular tachycardia in three, and asystole in one. Forty eight hours later the hearts were removed. Conventional electrophysiological techniques revealed the following: 1) injured cells had loss of resting potential, triangularization of the action potential and depressed upstrokes; 2) the functional refractory period was shortened, with an increase in time dependence; 3) action potential alternans; 4) discrete areas of marked depression of conduction; 5) re-excitation with premature stimulation and 6) histological evidence of sub-epicardial injury. These studies suggest that the application of suprathreshold shocks may induce electrophysiological changes potentially inducing tachyarrhythmias.