The standard concept of the renin-angiotensin system is that renin is secreted by the juxtaglomerular cells of the kidney into the circulation, where it cleaves angiotensin to release angiotensin I. The angiotensin I is converted to angiotensin II by a converting enzyme located on the plasma membrane of the endothelial cell. The released angiotensin II binds to receptors on target cells to initiate a series of intracellular actions that result in a specific cell function. The kidney was conceived to secrete renin with the circulatory angiotensin II returning to the kidney to alter renal function. It is now clear that all the components of the renin-angiotensin system can be synthesized within the kidney. In fact, angiotensin II is formed in very high concentrations in the renal interstitial space. Local angiotensin II production can have profound influences on renal function,ie, alter glomerular hemodynamics, reduce sodium excretion, and constrict small arterioles. In certain disease states, the local action of angiotensin II may have harmful effects on the kidney, and blockade of the renin-angiotensin system can be beneficial to the kidney.