Experiments have been performed to investigate laminar mixed convection heat transfer in a horizontal rectangular channel, with discrete heat sources at the upper and lower surfaces, and having an aspect ratio of W/H (width/length)=8. Each of flush mounted heat sources represents a chip in electronic devices. Side, bottom and top walls of the channel were insulated and the discrete heat sources were subjected to an uniform heat flux. Experiments were performed at different Reynolds and Grashof numbers. The steady-state flow conditions were determined to exist by observing the variation of temperature distribution at the top and bottom walls of the channel. For the bottom heat sources, the fluid flow over the first rows of heaters shows a forced-convection thermal entry region characteristic. For the last rows, however, the buoyancy-driven secondary flow causes an increase in the Nusselt number. The buoyancy-driven secondary flow becomes more effective with the increase in Grashof number. Although the top heat sources are seen to be affected from the air flow rising from bottom regions, they are more affected by the forced-convection fluid flow.