Abstract: The two preferred and illustrated embodiments set forth downhole tool cooling systems. In a downhole oil well tool exposed to extremely high working temperatures, a cooling system is provided for the printed circuit boards and various electronic components thereon. In one embodiment, the cooling system incorporates a closed container having a bottom located sump, a pump for picking up and pumping liquid refrigerant from the sump, and a distribution line extending from the pump to spray the PC boards with refrigerant. Cooling is accomplished on vaporization, and refrigerant flows back to the bottom as it condenses. In the alternate embodiment, no pump is included. Rather, a heat sink is included at the top of a closed and thermally isolated chamber. The bottom includes a sump for refrigerant. In between, a mesh distributes droplets of condensing liquid over the surfaces of the PC boards to accomplish cooling.