THE MOST COMMON failures of implanted cardiac pacemakers used in the treatment of heart block have been due to wire breakage, electrode detachment, or battery depletion. Electronic malfunction has occurred in 1% or less of implanted units.1,2This report documents ventricular tachycardia and ventricular fibrillation caused by electronic failure of an internal electrical pacemaker. Report of a Case A 64-year-old man had a fixed-rate pacemaker implanted in 1961 for treatment of complete heart block. Two years later, Oct 3, 1963, the battery unit required replacement because of battery depletion. On June 1, 1964, augmentation of the pacemaker output was necessary, due to increased resistance of one electrode. Testing of the battery unit revealed a normal output and rate with no evidence of a direct-current leak. Two weeks later, on June 14, the patient had the onset of intermittent syncopal episodes. Physical examination revealed a pale, anxious man. The blood