Experiments were performed using water flow in microchannel specimens, with thermocouples mounted axially along the flow for local wall temperature measurement. The processed experimental data obtained locally in the laminar regime exhibited the unusual behaviour of the local Nusselt number decreasing with increasing local Reynolds number along the flow. Though the variation of Nusselt number with Reynolds number has been observed before, it was for averaged parameters, obtained using a different approach in experimentation for calculating these parameters over the entire microchannel specimen. The present experimental data in the laminar regime were also found to correlate well with the Brinkman number for the individual sets and much better globally, by combining all the data sets. The correlation with the Brinkman number in spite of its relatively low values is physically interpreted. (C) 2000 Elsevier Science Ltd. All rights reserved.