In this report an asymptotic approach is developed for estimating the aqueous layer permeability in a perfused rat intestine experiment. The analysis is based on the technique presented by Gottifredi and Flores (1985) for studying the heat transfer rate between a flowing non-Newtonian fluid and the walls of a duct. The aqueous layer permeability is expressed as a series solution of the Graetz number. Our results are compared with previous estimates presented in the recent literature and the agreement is excellent in the low-Graetz number region.