A mechanical auxiliary ventricle for intermittent permanent assistance was implanted Feb 4, 1966, in a 33year-old man with intractable left ventricular failure due to idiopathic cardiomyopathy. When the ventricle began functioning, systemic pressure was returned to normal as was left auricular pressure. During the immediate postoperative period the patient maintained fair stability with arterial pressure remaining at physiologic levels. The patient's sensorium appeared normal postoperatively. His activity required sedation. No unusual sensations related to the implantation were reported. Despite a good clinical picture, a constantly increasing pulse rate rose to 160 to 180 beats per minute. Eleven hours postoperatively his left auricular and central venous pressure rose sharply while peripheral arterial pressure fell. The patient died 20 hours and 30 minutes following completion of the operation. Cause of death is not fully understood but may be related to prolonged uncontrollable tachycardia.