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Purpose. To evaluate the use of an in vitrointestinal permeability model to predict rat and human absorption as well as to evaluate the use of an internal standard to control for intra- and inter-rat variability. Methods. In vivoperoral absorption and in vitro steady-state intestinal permeability coefficients were determined in the rat for a variety of structurally different compounds with different physicochemical properties including: progesterone, hydrocortisone, salicylic acid, caffeine, clonidine, p-aminoclonidine, UK-14304, oxymetazoline, mannitol, PEG 900, PEG 4000, and a number of novel hydrophilic chemical entities. Results. The intestinal permeability coefficients determined in vitro could be used to predict the peroral absorption of a compound in both the rat and human. Normalizing the permeability of a test compound to an internal standard, e.g. mannitol, greatly improved the prediction of peroral absorption. Conclusions. The use of an internal standard can aid in the prediction of the peroral absorption of a test compound, in particular, for one that has moderate absorption in the range of 20−80%. Moreover, these methods would appear to be a useful means to improve the prediction of other absorption models as well, such as the Caco-2 cell systems and in-situ perfusion methods.