Introduction The widespread laboratory interest and increasing clinical acceptance of blood stream hypothermia have led to a strong demand for equipment to produce rapid temperature changes in the blood of animals and humans. The apparatus described by Brown et al. is relatively satisfactory and has received widespread use despite the fact that it has a large priming volume, a potentially dangerous design, and is expensive.1 A review of characteristics desirable in a heat exchanger would appear timely (Table 1). An apparatus incorporating these features has been in use, experimentally and clinically, for approximately 2 years.3 Description of Apparatus Basically the heat exchanger consists of two 4×12 in. opposing plates of siliconed stainless steel separated by a 1/16 in. Teflon gasket. The inlet and outlet are formed from stainless steel pipe having an internal diameter of 1/4 in. The gasket is held tightly by a series of bolts and