We have developed an implantable pneumatically driven diaphragm left ventricular assist system (LVAS) for small adults, and an electric impedance based monitoring and control system (Z system). In this Z system, measurement of electric impedance of the blood chamber is performed by charging an alternating current with a constant amplitude between two metal connectors without compromising anti-thrombogenicity. Automatic calibration was performed periodically by measuring trough values of impedance in the blood chamber for several beats while the blood pump was in a full-fill state under preset driving conditions. For precise control of the fill-to-empty (F/E) drive, an automatic trigger level adjustment method was developed. This method is an automatic searching algorithm that identifies the volume change of the last ejection or filling by the Z system and settles the next trigger level to maintain a definite stroke volume. In chronic in vivo evaluation, this LVAS was used in 13 adult goats. The pump was installed between the LV apex and the descending aorta and was placed in the abdominal wall. Pump output ranged from 2.5 to 5.8 L/min, and the Z system provided good monitoring. The F/E drive was achieved satisfactorily by this Z system. The physical condition of the goats was excellent for as long as 19 weeks. Hematologic, hepatic, and renal functions were within the normal range, and no prominent thromboembolic symptoms were observed. A new mobile control drive unit with this Z system is being developed. We conclude that this LVAS with the Z system is promising for long-term use in a clinical setting.