The components of the plasma RAS originate primarily from renal renin, hepatic angiotensinogen, and pulmonary converting enzyme (Fig. 1.2). However, recent progress in biotechnological and immunocytochemical techniques enables us to detect trace amounts of biologically active peptides and their mRNA, which facilitates discovery of each component of the RAS in various tissues which were not thought to be the sites of production. Since some of these tissues such as brain, kidney, heart and blood vessels produce all components of the RAS (see Crmpbell 1987a; Dzau 1988; Baker et al. 1992; Saavedra 1992 for reviews; Fig. 1.3; Table 4.1), a paracrine or autocrine function of ANG II has to be taken into account when considering its physiological action.