Renal kallikrein is synthetized in the distal nephron and secreted in urine. Studies on the microdissected nephron and on the whole kidney using immunohistochemistry indicate that kallikrein is produced at the level of the distal part of the distal tubule and in some species in the cells of the initial part of the collecting duct. Urinary kinins are formed at these same sites. The renal tubule contains high levels of kininases, able to inactivate kinins, but the kininases are located mainly up-stream the sites of kinin production, i.e. in the glomerulus and in the proximal tubule. A kininase activity is however also found in the medullary collecting duct. This collecting duct kininase activity might contribute to the control of kinin levels in kidney and urine. The physiological role of the renal kallikrein kinin system is not completely understood. There is some evidence for a role of urinary kinins in water and solute transferrence in the distal nephron. Also kinin production in the renal interstitium and/or circulation may contribute to the control of renal blood flow. In the isolated perfused rat kidney vasoconstricted by angiotensin II for example, the high molecular weight kininogen is hydrolyzed to release kinins and induces a vasodilatation. The kallikrein-kinin system therefore might play a role in kidney function by antagonizing the vascular effect of angiotensin II in the renal circulation and the maintenance renal blood flow.