Single-phase, natural convection heat transfer data have been obtained for an array of highly finned, discrete heat sources mounted to one wall of a cavity filled with a dielectric liquid (FC-77). Dense, parallel plate fin arrays were considered for both vertical and horizontal cavity orientations, and the finned surfaces were found to enhance heat transfer by as much as a factor of 24. Thermal resistances approaching 2 cm(2) degrees C W-1 were realized, while maintaining the temperature difference between the fin base and an opposing cold plate below 70 degrees C. In a parallel numerical study, how and heat transfer conditions were calculated by treating the fin array as a porous medium characterized by the Brinkman-Forchheimer-extended Darcy model. The model provided a reasonable approximation which captured the experimental trends and predicted the data to within 30%.