Over the past 6 years, research has led to development of a small, lightweight, power-efficient, uniquely simple ventricular assist device driven by a magnetic actuator. Magnetic actuation permits total elimination of all mechanical motion converter components used for pusher plate displacement, resulting in a significant reduction in complexity and resultant increase in reliability. Extensive in vitro mock loop development has resulted in a left ventricular assist device (LVAD), the primary design parameters of which for the clinical prototype actuator and pump are 1) an actuator weight of 312 g, 2) actuator size of 32.5 cm3, 3) power requirements of 7.8 to 11.4 watts (60-100 beats per minute [BPM]), and 4) system efficiency of 24% to 34% and average dynamic stroke volume of 65 ml. Initial in vivo tests assessed this LVAD's performance in four sheep under three acute conditions of ventricular dysfunction. The results demonstrate that, at a pump-rate of 100 BPM, mean aortic pressure increased by 45-50 mmHg during 1) beta blockade, 2) coronary ligation, and 3) ventricular fibrillation. Pump flow ranged from 2.71 L/min to a maximum of 4.6 L/min. Acute test periods were arbitrarily set for 6 hours duration. Of the four sheep, two survived, one lived 5 hours, and the fourth lived 4.5 hours. Global fibrillation was the primary cause of failure. This initial in vivo data demonstrates the pump's ability to maintain satisfactory hemodynamic parameters of flow and pressure under three acute conditions of extreme left ventricular dysfunction in an animal model. These initial LVAD performances were encouraging. Further tests will use calves with a greatly expanded performance evaluation protocol.