The chapter highlights the use of perfusion of liver and other organs for the study of microsomal electron-transport and cytochrome P-450 systems. Perfusion of organs has become a widely used experimental technique for the study of metabolic properties in general, including the state and activity of microsomal electron-transport and cytochrome P-450 systems. The principal advantage of the perfused organ as a complex metabolic model resides in the possibility of investigating steady states in an open metabolic system, in contrast to closed metabolic systems, like isolated cell incubations, which have advantages in other respects. The liver is the organ that is most intensively used for perfusion studies, and the perfusion of other tissues—for example, the lung, kidney, and intestine, as well as steroidogenic organs may become of increased interest for the study of microsomal activity at a more complex level of metabolic organization. The isolated perfused small intestine of the rat is a valuable metabolic model of the intestinal mucosa cells and of the partitioning between lumen, blood, and lymph. A well characterized preparation consists of perfusion via the superior mesenteric artery with heparinized blood. A hemoglobin-free perfusion also is of metabolic competence, 6° particularly if fluorocarbon emulsions are present for oxygenation.