Ten acute and chronic in vivo studies were utilized to develop a pneumatic ventricular assist device (VAD) as a bridge to heart transplantation or a circulatory assist device for patients with end-stage heart disease or poor myocardial funct on after cardiac surgery. Two sizes of blood pump of 70cc for adult patients and 34cc for pediatric patients were implanted in the animals. Ventricle of the blood pump was made from the polyurethaae to enhance antithrombogenecity. The VAD was implanted between the left atrium and the descending aorta. Average flow rate was 2.38 L/min for adult and 0.41 L/min for pediatric VAD at the rate of 60 bum. The duration of support ranged from 1 to 26 hours. The most frequent complication was bleeding. Main causes of death were heart failure and respiratory failure. The device function was good for short term use. Studies to date suggest that, with further refinement, a reliable long term VAD that will have clinical application can be developed.