The anatomy of the intestine is outlined. The kinetics of cell proliferation, differentiation, and migration in intestinal epithelium are described. The distribution of cell types between the proliferation and differentiation compartments of the epithelium is summarized. Seven preparations of intestine in current use for metabolic studies are listed. An outline review is presented of recent results on glucose, amino acid and ketone body metabolism obtained with three of these preparations: the ligature-isolated loop of intestine in vivo; the vascularly and luminally perfused intestine in vitro; and suspensions of respiring intestinal epithelial cells in vitro. Reasons are given for thinking that these experimental systems are among the most useful currently available for metabolic studies, perhaps especially in the investigation of the metabolic fate of potentially toxic materials to which animal species may be exposed.