This chapter describes the issues concerning the gastrointestinal (GI) tract as a route of drug administration and the associated processes of drug absorption, bioavailability, and distribution. The function of the GI tract is the ingestion of foodstuffs which are subsequently broken down into basic nutrients for absorption for general sustenance. Pharmaceutical companies search for effective drugs that possess a high potential to be absorbed in the GI tract, are well distributed in the circulatory system, and are not metabolized before it can reach the ultimately the receptor site. The gastrointestinal tract present in multi-cellular organisms that have the capability of providing nourishment to the organism to maintain life functions. Portion of the GI tract is highly specialized and in conjunction with the accessory digestive organs that are connected through a series of ducts. Pharmacokinetics generally refers to the mathematics of movement of chemical substances through the body including the way in which the body's physiology affects the substances over time.