This paper reports the results of a numerical study of the Nusselt number for the H1 and T boundary conditions in laminar, fully-developed flows of pseudoplastic and dilatant fluids in rectangular ducts. Equations for the apparent viscosity that span the entire shear rate range were utilized and the Nusselt numbers were calculated as a function of a shear rate parameter that determines the flow regime where the duct is operating. Numerical results for the Nusselt number in all flow regimes are included along with correlation equations. Finally, errors associated with applying power law solutions are discussed. (C) 2009 Elsevier Ltd. All rights reserved.