The primary function of the small intestine is to absorb nutrients and water. This is achieved by mixing food with digestive enzymes to increase the contact of foodstuffs with the absorptive cells of the mucosa. In humans, the small intestine is about 5 to 6 m in length and comprises approximately 1% of body weight (ca. 0.7 kg for adults), which is significantly smaller than the liver (ca. 1.5 kg for adults). Approximately 6 to 12 liters of partially digested foodstuffs, water, and secretions are delivered daily to the small intestine. Of this, only 10 to 20% are passed on to the colon, because most nutrients, electrolytes, and water are absorbed as they are transported through the small intestine. Absorption and movement of the contents are brought about by the activities of the absorptive cells of the mucosa and by coordinated contraction of the smooth muscle cells of the muscularis extern (Weisbradt, 1987; Guyton, 1991). In addition to this fundamental role, a secondary function of the small intestine arises from the fact that it is also a major route of entry into the body for many xenobiotics including drugs.

Although the small intestine is regarded as an absorptive organ in the uptake of orally administered drugs, it also has the ability to metabolize drugs by numerous pathways involving both phase 1 and phase 2 reactions (Caldwell and Marsh, 1982; Renwick and George, 1989; Ilett, 1990; Ilett et al., 1990; Krishna and Klotz, 1994). Anatomically, the small intestine has a serial relationship with the liver relative to the absorption and is the anterior organ. The amount of an orally administered drug that reaches the systemic circulation can be reduced by both intestinal and hepatic metabolism. The metabolism of drugs before entering the systemic circulation is referred to as first-pass metabolism. It has been