INFOBOX Human effective intestinal permeability (Peff), is a major biopharmaceutical variable, and together with luminal drug stability, drug luminal solubility, luminal dissolution rate, and GI transit rate, it strongly influences the in vivo rate and extent of intestinal drug absorption in both humans and animals. Direct experimental in vivo approaches in humans and animals to examine intestinal drug absorption, secretion, and metabolism are feasible by applying regional intestinal perfusion techniques. In general, four different types of clinical GI tube have been employed in the small and large intestine to various extents. A significant correlation has been established between in vivo intestinal Peff and pharmacokinetically derived values for the fraction of dose absorbed for structurally different drug molecules.