The single-pass perfused rat intestinal model is an in situ perfusion method that can be used to determine regional disposition of drugs. It is useful for selecting a development candidate from a series of active compounds and for studying mechanisms of absorption and excretion. It is also useful for determining if a compound may be appropriate for a sustained-release control formulation. The Food and Drug Administration (FDA) recognized the model system as a useful model to classify a compound’s absorption characteristics in the Biopharmaceutics Classification System. This model may be modified to determine the contribution of intestine versus liver in the disposition of a specific compound, both of which may be useful to determine the enteric and enterohepatic recycling of drugs.