Introduction: the gastrointestinal (GI) tract as a barrier and as an absorptive and metabolic organ / Shayne C. Gad -- Methods for the analysis of gastrointestinal function / Robin C. Guy -- Safety pharmacology and the GI tract / Henry I. Jacoby -- Gastronintestinal tract development and its importance in toxicology / Joseph V. Rodricks, Allison A. Yates, and Claire L. Kruger -- The GI tract as major route of pharmaceutical administration / Robert W. Kapp, Jr. -- Gastrointestinal function and toxicology in canines / Charles Spainhour -- Absorption of macromolecules by mammalian intestinal epithelium / Shayne C. Gad -- Peyer's patch epithelium: an imperfect barrier / Gary R. Burleson and Florence G. Burleson -- Alteration of intestinal function by xenobiotic exposure: animal models / Shayne C. Gad -- Intestinal absorption and metabolism of xenobiotics in laboratory animals / William J. Brock -- Normal and abnormal intestinal absorption by humans / David W. Hobson and Valerie L. Hobson -- Intestinal absorption and metabolism of xenobiotics in humans / William J. Brock and David W. Hobson.