A major problem in maintaining perfusate temperature is the heat loss that occurs during transfer of the fluid through tubing on its way to the heart. This heat loss can cause changes in temperature of several degrees centigrade compounded due to the effects of changes in perfusate flow caused by coronary regulation. To correct this problem, the transfer tubing is jacketed using our Flex Tube Assembly, which provides a tube within a tube configuration. The outer tubing connector is a glass adapter with a Q.D. side port for water entry/exit and threaded neck. The connection for the central transfer tubing is constructed of 6mm O.D. glass tubing inserted through a polypropylene cap and O-ring that screws onto the outer tube adapter. One end of the inner glass tubing is a female luer fitting and the other end tapers to 3mm O.D. over which the Tygon tubing is fitted. The outer Tygon tubing is 3/8 ID by 1/2 O.D. Besides its use in the isolated heart perfusion system, this assembly has uses in many applications that require stringent temperature control.