Direct current electroshock is an effective means of correcting atrial flutter or fibrillation. As recent reports stress the safety of this method, we are recording an unusual reaction as indicated by the electrocardiogram ( ECG), paralleling the monitored image on the oscilloscope. The patient who had had acute rheumatic fever suffered from chronic atrial fibrillation. A trial with DC shock was undertaken because of sensitivity to procainamide. Three shocks administered without anaesthesia did not restore normal rhythm. Six days later, DC shock was followed by regular sinus-dominated rhythm which coincided with the appearance of striking elevation of the S-T segment of the ECG, reminiscent of early myocardial infarction or a variant form of angina pectoris. There were no troublesome sequelae.