The transvenous automatic defibrillator is a device which automatically recognizes and treats ventricular fibrillation. The initial clinical prototype resembles an external pacemaker: the electronic components and power supply are external to the body, the sensing and defibrillating elements being contained in a transvenous catheter. The sensors monitor intracardiac R waves and cardiac contraction. Absence of both signals identifies the arrhythmia and triggers the defibrillatory shock, delivered through the right ventricular electrode and another more proximal on the catheter. If needed, the device recycles automatically. It is also adaptable for pacing and for elective intraatrial cardioversion. Results of animal experiments were successful, and tests are currently being done on patients requiring defibrillation during open heart surgery. The clinical indications for the device are numerous and it is hoped that this approach will decrease the present prohibitive mortality from coronary heart disease.