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We are studying in vivo an intraventricular axial flow blood pump (Jarvik 2000) designed for long-term left ventricular support. The small (25 cc, 85 g) valveless pump has been placed intraventricularly in seven calves; pumps have functioned for as long as 5 months. In the four most recent longterm studies completed, calves have survived for 70, 120, 155, and 162 days (in that order); weight gain has averaged 0.56 kg/day. One study is ongoing at more than 30 days. Under resting physiologic conditions in the normal calf, the continuous flow pump produces flows of 5–6 L/min with a decreased arterial pulse contour. The device has caused no physiologic complications. Calves in the completed studies had mean free plasma hemoglobin levels of 11.4, 7.1, 6.5, and 4.3 mg/dl, respectively. We have modified the inflow structures of the device, and these results suggest that a thrombus free design with no pannus at or around the inlet of the pump can be achieved. Histopathologic analyses of the heart and kidneys in studies of as long as 5 months show no deleterious effects of this device. These studies demonstrate the feasibility of a small implanted intraventricular blood pump for long-term use. Future developments for permanent implantation will include implanted physiologic control systems, transcutaneous energy transmission systems, and implanted batteries.