The object of this investigation is to obtain qualitative and quantitative understanding of reentrant hole (or extraction channel) effects in pulsed thermal- and fast-neutron experimental assemblies. The calculational model used assumes slab geometry for the unperturbed (without the hole) situation and considers cylindrical reentrant holes of various diameters and depths. The two-dimensional nature of the hole is represented by a wall-streaming term which is used as a boundary condition for a reduced effective slab. The effective slab geometry is obtained by reducing the thickness of the original slab by an amount equal to the depth of the reentrant hole. The validity of this important simplification is confirmed by results of two-dimensional discrete ordinates transport calculations in which the reentrant hole is introduced explicitly. A second basic assumption used to simplify the numerical calculations is that the flux along the walls of the reentrant hole is adequately represented by the unperturbed flux. This approximation is judged valid by the success of the method in predicting experimental results. The analytical procedure is applied numerically using discrete Sn transport theory. Solutions are obtained from a code system which makes use of a standard production program DTF-IV as a subroutine for performing unperturbed and perturbed effective slab calculations. The calculational model yielded good predictions of the distorted fluxes for reentrant hole experiments performed on water at Rensselaer Polytechnic Institute. For fast neutron spectra, the model predicted distortions (particularly at high energies) which were significant but not large enough to limit the viability of the experiment.