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The rate and extent of absorption of orally ingested compounds are influenced by properties that are intrinsic to ingested substances themselves as well as factors that are associated with the milieu of the alimentary canal and its absorptive surface. This chapter presents comparative, quantitative data for the previously identified important anatomical and physiological parameters of the gastrointestinal tract for several additional mammalian species. Data for the dog, rabbit, mouse, pig, and monkey, and updated information for rats and humans is presented. It has been found that perfused jejunal segments of normal human subjects absorb fluids at a rate that is 5–10 times greater per unit area of mucosa than that of laboratory rats. It is emphasized that the parameters described relate to the crossing of materials from the gastrointestinal lumen into the bloodstream. The information about absorption …