The rate at which ingested compounds are absorbed into the bloodstream from the gastrointestinal (GI) tract and extent to which they are absorbed are governed by the physical‐chemical properties of the ingested compounds, the presence and identity of other substances in the GI tract, and the anatomical and physiological characteristics of the GI tract. This chapter describes the anatomical and physiological similarities and differences between humans and some common laboratory animals, with an emphasis on the absorptive surface area of the GI tract, the time it takes for materials to move through the GI tract, and the nature of the GI contents.