The direct broadcast satellite (DBS)* being developed by RCA Astro-Electronics Division for the Satellite Television Corporation (STC) is the first domestic commercial direct broadcast satellite. The high power dissipation levels of the payload's three 220W transponders present a unique challenge to the thermal engineer. Preliminary design quickly confirmed that heat pipes were necessary to “isothermalize” the payload panels. Design features of the selected heat pipes and the unique testing program to validate their performance is reviewed. The mission requirement for no payload eclipse operation and its impact on the thermal design is discussed, including the transient response characteristics required for rapid resumption of operation following emergence from earth eclipse. The satellite testing program is reviewed in detail. Comparison between analytical predictions and test data is provided.