The convective laminar flow through a horizontal rectangular duct, with significant buoyancy effects, has been investigated analytically and numerically, employing the constant property (CP) model and the variable property (VP) model. The duct has a finite heated region on the bottom and three-dimensional transport is studied. The CP model employs the Boussinesq approximations and uses properties evaluated at four different reference temperatures, i.e. the ambient temperature T-0, the average temperature T-f, the integrated average temperature T-int, and the heat source temperature T-h. In the VP model, the density and the transport properties are computed using the state equation of an ideal gas and power law correlations, respectively. Numerical results for temperature ratio epsilon, where epsilon = (T-h - T-0)/T-0, ranging from 0.033 to 2.33 are presented. The spanwise variation of the transport quantities is investigated in detail for the different models, and several interesting and important tends are obtained. These results will be of considerable value in model development for a wide variety of thermal systems and processes, where large changes in material properties are encountered.