Combined free and forced convection for steady fully developed laminar flow in horizontal rectangular channels under the thermal boundary conditions of axially uniform wall heat flux and peripherally uniform wall temperature at any axial position is approached by the method of successive overrelaxation. The convergence of the numerical solution is ascertained. Graphical results are presented for streamlines, isotherms, w/w0 versus Re Ra, f Re/(f Re)0 versus Re Ra, and Nu/Nu0 versus Re Ra for the aspect ratios γ = 0.2, 0.5, 1, 2, and 5 and Pr = 0.73. For square channels, velocity and temperature distributions for Pr = 0.73 and heat transfer results for Pr = 7.2 are also presented.