Nitrogen oxide (NO) is one of the crucial modulators of the vascular tonus. Apart from its effect on the cardiovascular system it exerts an effect also on other types of cells and ensures their functions.Specially comprehensive is its synthesis and action in the kidneys: NO is formed in the endothelial cells due to the activity of constitutional endothelial synthase (eNOS), in mesangial cells of inductive synthase (iNOS), in smooth muscle cells (vsmNOS), in tubular cells neuronal NOS (nNOS) and iNOS and in the macula densa nNOS. By modulation of the v.afferens it influences the blood flow through the glomeruli and filtration pressure in the glomeruli. It participates in the tubuloglomerular feedback: the cells of the macula densa produce NO via nNOS, the genetic transcription and translation of which as well as the kationic translation system ensure the transport of the L-arginine precursor and regulate very sensitively NO formation. The latter diffuses via the extraglomerular mesangium into the iuxtaglomerular apparatus where renin is forned.NO reduces proteinuria and renal proliferation. During renal insufficiency NO production is inhibited and in diabetes NO production is increased. Diabetic hyperfiltration and hypertrophy are ascribed to produced NO. Experimental studies contributed substantially to the knowledge of renal effects of NO. At present intensive clinical research has been started which, no doubt, will influence medical practice.