The flow characteristics of a network of parallel microchannels (hydraulic diameter: 110 mu m) are investigated both experimentally and numerically. The microchannel cross-section was triangular as in the case of microheat pipes. The pressure drop across the microchannel network showed a dramatic increase with a departure from the law of fully developed flow in ducts as soon as the Reynolds number of the flow exceeded about 10. Numerical computation of the flow was carried out using the classical laws of hydrodynamics in an attempt to explain this surprising result. There was a good agreement with experimental results, which suggests that there are no size effects at the length scales used in the experiments. Moreover the mechanisms responsible for the large pressure drop for higher Reynolds numbers were identified in the numerical analysis as being extra head losses due to separation in several parts of the test section.