Low energy endocardial cardioversion was attempted in 23 patients with 30 arrhythmias, of whom only four were receiving additional drug treatment. Four had atrial flutter, five atrial fibrillation, three intra-atrioventricular nodal tachycardia, two atrioventricular re-entrant tachycardia, 13 ventricular tachycardia, and three ventricular fibrillation. A pacing lead with special large surface area electrodes--the active electrode positioned either in the right atrium or in the right ventricular apex and the indifferent electrode in the right atrium, superior vena cava, or inferior vena cava--was used together with a low energy defibrillator. A total of 114 shocks was delivered, 26 of which were atrial. One episode of atrial flutter was terminated, but atrial fibrillation and atrioventricular nodal tachycardia were not terminated in any of the patients. Both patients with atrioventricular tachycardia were successfully treated, as were eight of the patients with ventricular tachycardia. Atrial fibrillation was produced in three patients and non-sustained ventricular tachycardia in one, ventricular tachycardia was accelerated in two, and ventricular fibrillation induced in five. Fourteen patients experienced severe discomfort and seven mild or moderate discomfort, and only one found the procedure painless. One patient was anaesthetised throughout the procedure. Low energy endocardial cardioversion is not universally successful even at the highest energies tolerable, and with the present electrode and pulse waveforms some patients may suffer considerable discomfort.