Ventricular defibrillation was acheived in active conscious dogs with a chronically implanted automatic system composed of a defibrillator and an alternating current fibrillator. The hermetically sealed defibrillator is encased in titanium, weighs 250 g and has a volume of 145 ml. The sensor continuously monitors ventricular electircal activity and recognizes fibrillation by the absence of isoelectric potential segments. Fibrillation is induced by placing a magnet over the implanted fibrillator. The resulting syncope closely resembles the clinical entity of sudden death, while the defibrillator automatically restores normal rhythm with a truncated exponential pulse of 30 J, 15 seconds after the onset of the arrhythmia. The operational status of the defibrillator can be tested in vitro and noninvasively in vivo with an external analyzer. This experimental model allows for the first time a long-term study of the automatic implantable defibrillator approach to prevent sudden death from ventricular fibrillation under a variety of physiopathologic conditions.