The development of the laminar velocity and pressure fields in the hydrodynamic entrance region of rectangular ducts has been explored experimentally. Duct cross sections having aspect ratios of 5:1 and 2:1 were employed in the investigation; air was the working fluid. It was found that the development of the pressure field is much more rapid than that of the velocity field. The entrance length, relative to pressure development, is representable as (z/De )/Re = 0.02 for both ducts. The incremental pressure drop due to the development of the flow was deduced from the experimental data as being approximately equal to one velocity head. The axial development of the velocity field is illustrated by a sequence of velocity profiles measured along the symmetry lines of the cross section. The flow development in the 5:1 duct is found to be somewhat more rapid than in the 2:1 duct. Comparisons of the experimental results are made with available predictions of analysis, all of which are based on approximate models of the flow field. In general, the analyses over predict the incremental pressure drop due to flow development. The development of the velocity field appears to be reasonably well described by analysis.