The renin-angiotensin system (RAS) constitutes an important hormonal system in the physiological regulation of blood pressure. The dysregulation of the RAS is considered a major influence in the development and progression of cardiovascular disease and other pathologies. Indeed, experimental and clinical evidence indicates that blockade of this system with angiotensin-converting enzyme (ACE) inhibitors or angiotensin type 1 receptor (AT 1R) antagonists is an effective therapy to attenuate hypertension and diabetic renal injury, and to improve heart failure. Originally defined as a circulating system, multiple tissues express a complete RAS, and compelling evidence now favors an intracellular system involved in cell signaling and function. Within the kidney, intracellular expression of the three predominant ANG receptor subtypes is evident in the nuclear compartment. The ANG type 1 receptor (AT 1R) is coupled to the generation of reactive oxygen species (ROS) through the activation of phosphoinositol-3 kinase (PI3K) and PKC. In contrast, both ANG type 2 (AT 2R) and ANG-(1-7) (AT 7R) receptors stimulate nitric oxide (NO) formation, which may involve nuclear endothelial NO synthase (eNOS). Moreover, blockade of either ACE2-the enzyme that converts ANG II to ANG-(1-7)-or the AT 7 receptor exacerbates the ANG II-ROS response on renal nuclei. Finally, in a model of fetal programmed hypertension, the nuclear ROS response to ANG II is enhanced, while both AT 2 and AT7 stimulation of NO is attenuated, suggesting that an imbalance in the intracellular RAS may contribute to the development of programming events. We conclude that a functional intracellular or nuclear RAS may have important implications in the therapeutic approaches to cardiovascular disease. renin-angiotensin system; angiotensin converting enzyme; ACE2; neprilysin; kidney; nuclei; AT 1 ; AT 2 ; AT 7 , FROM THE EARLY STUDIES OF Tigerstedt and Bergman (101) that described renin activity in the kidney to the characterization of a pressor substance subsequently identified as ANG II by the laboratories of Braun-Menendez (6) (hypertensin) and Page (angiotonin) 50 years later (8), the characterization of the renin-angiotensin system (RAS) within the kidney and other tissues continues unabated to broaden our understanding of the functional aspects of this important hormonal system in blood pressure regulation and cardiovascular pathologies. Coupled with the discoveries of ANG I-converting enzyme (ACE) as the primary enzyme that forms ANG II in the circulation and tissue and the molecular identification of the ANG II type 1 receptor (AT 1 ) that confers the predominant actions of ANG II, the "classical" RAS was regarded as a sequential endocrine system to form ANG II for the maintenance of blood pressure through renal, vascular, and central mechanisms. There is, however, overwhelming evidence for a "nonclassical" RAS that results in the formation of novel peptide products with functional properties distinct from that of the ANG II-AT 1 receptor pathway (5,12,14,67,8...