Implantable cardiac defibrillators are now an accepted form of therapy for patients with life-threatening ventricular arrhythmias that cannot be controlled by antiarrhythmic drugs. These devices could be made even more acceptable if they were smaller, had increased longevity and the surgical procedure for implantation was less invasive. Reducing the energy requirements for internal defibrillation with use of a nonthoracotomy lead system would make all of these goals achievable. Monophasic and double and single capacitor biphasic waveforms were compared in 14 anesthetized dogs (25.5 ± 2.2 kg) with use of a nonthoracotomy lead system that has previously been shown to distribute the delivered voltage throughout the heart more equally. Cathodal catheter electrodes were placed in the right ventricular apex and outflow tract. The anodal electrode was a large cutaneous R2 patch placed over the left side of the chest. The mean energy requirement for defibrillation when a single capacitor biphasic waveform was used was significantly less (6.4 ± 2.6 J) than that for either the double capacitor biphasic or the monophasic waveform (18.0 ± 8.0 and 17.4 ± 8.0 J, respectively) of the same duration. Unexpectedly, the leading edge voltage for the phase I of the single capacitor biphasic waveform was significantly less (266 ± 51 V) than that for either the double capacitor biphasic or the monophasic waveform (336 ± 76 and 427 ± 117 V, respectively). In conclusion, in large dogs, defibrillation is possible at low energy levels with a single capacitor biphasic waveform. © 1989.