Ventricular fibrillation and other tachyarrhythmias are controlled by delivery of two sequential impulses to two pairs of electrodes placed partially within or adjacent to the heart. One electrode pair is provided by an endocardial lead which includes a distal electrode adapted to reside in the apex of the right ventricle and a proximal electrode designed to lie in the superior vena cava. The second electrode pair comprises the distal electrode within the right ventricle and a further electrode lodged in or adjacent to the coronary sinus. The metal case of the implantable pulse generator which supplies pulses to the electrode pairs may be substituted for the coronary sinus electrode if the pulse generator is implanted above and to the left of the heart in the left pectoral region of the patient's body. Alternatively, a subcutaneous plate electrode may be substituted for the can, obviating the need for implanting the generator in the left pectoral region. The system also works for temporary use with a skin electrode pair with the catheter in conjunction with an external defibrillator. This disposition of electrode pairs produces a temporal and spatial summation of the delivered energy which results in a reduction in the energy required for defibrillation or cardioversion, and allows the use of less traumatic surgical procedures than epicardial lead systems.