A boundary layer approach is developed for estimating the aqueous resistance in a perfused rat intestine experiment. Knowing the aqueous resistance allows the membrane surface concentration to be calculated as a function of the perfusate inlet concentration and perfusional flow rate. Determination of membrane uptake as a function of the membrane surface concentration rather than the perfusate concentration gives the intrinsic, unbiased membrane parameters for the uptake mechanism of Michaelis-Menten-type kinetics in parallel with passive diffusion. The aqueous resistance derived in the analysis is verified by comparison with flux data for 1-leucine and progesterone measured at various flow rates and intestinal lengths. The approach allows for a direct estimate to be made of the unbiased membrane permeability parameters.