Experiments were performed in a flat rectangular duct to determine the heat transfer and pressure drop response to periodic, rod-type disturbance elements situated adjacent to one principal wall and oriented transverse to the flow direction. In a portion of the experiments, heat transfer occurred only at the rodded wall, while in the remainder, heat was transferred at both principal walls of the duct. Highly detailed axial distributions of the local heat transfer coefficient were obtained. These distributions revealed the rapid establishment of a periodic (i.e., cyclic) fully developed regime as well as recurring local maxima and minima. Cycleaverage, fully developed heat transfer coefficients were evaluated and were found to be much larger than those for a smooth-walled duct. Linear pressure distributions were measured between periodically positioned stations in the fully developed region, and the corresponding friction factors were several times greater than the smooth-duct values. The heat transfer and friction data were very well correlated using parameters that take account of the effective surface roughness associated with the disturbance rods.