The results of a numerical investigation on the effects of the temperature dependence of viscosity and thermal conductivity in simultaneously developing laminar flows of liquids in straight ducts of constant cross sections are used to obtain new correlations for the axial distributions of the peripherally averaged local Nusselt number. Three different cross-sectional geometries are considered, corresponding to both axisymmetric (circular and concentric annular) and three-dimensional (square) ducts. Uniform heat flux boundary conditions are specified at the heated walls. Viscosity is assumed to vary with temperature according to an exponential relation, while a linear dependence of thermal conductivity on temperature is assumed. The other fluid properties are held constant. A superposition method is applied to obtain the values of the peripherally averaged local Nusselt number by separately considering the effects of temperature-dependent viscosity and those of temperature-dependent thermal conductivity.