A heat exchanger for a totally implantable heat driven LVAD is an essential element in overall system thermal management. The heat exchange is accomplished by supplying cooling water from a pusher-plate driven water pump to the engine and then to the heat exchanger on the pump housing. The temperature of the interface between the blood and pump surface is of critical importance for clinically acceptable operation of the system. Temperatures were measured by instrumenting a pump housing with thermocouples and an electric heater on a mock circulatory loop. Flows were varied from 1 to 8 l/min and heat input was 20 watts. At 1.0 l/min pump flow the maximum inner surface temperature rise is 4.5 degrees C. In vitro tests were conducted to examine the effect of elevated temperature on platelete function. Both platelet aggregation and adhesion were reduced at elevated temperatures of 42 and 47 degrees C indicating a potential benefit of reduced thrombogenesis on the heated housing surface.