A downhole tool configured for conveyance within a wellbore extending into a subterranean formation, the tool comprising an electronics system and a heat-dissipating apparatus. The electronics system includes a controller, a memory, and surface communicating means, at least one of which is a heat-generating source. The heat-dissipating apparatus includes: a chassis engaging the heat-generating source and having a fluid passageway allowing fluid flow therethrough; a radiator for further heat dissipation; a pump; sensors to measure temperature of the chassis and the wellbore; and a compensator to regulate the pressure of fluid in the passageway.