Experiments on calves using two types of systems for nuclear left ventricular assist pumps are described. These two systems use a nuclear heat source containing 120 g, 1600 Ci Pu-238 in the form of plutonium oxide. The heat source for the first system is the tidal regenerator engine with no valve or seal and is controlled using a hall effect displacement sensor. The second left ventricular assist system uses heat generated by nuclear decay for producing high gas pressure which is converted into a high hydraulic pressure for pumping. The engine efficiency amounts to 7 to 16%, and the measurement value of the pump efficiency obtained is 85%. How these two systems are synchronized with the left ventricle of vital body is described. IN a transplant experiment for a short period (175 hours) in the initial stage, the possibility of the present method was demonstrated. A long-term performance experiment and improvement of the systems are anticipated. However, there still remain many problems, such as improvement of efficiency, reliability, and performance and diminution of the size.