Angiotensin II (Ang II) controls renal plasma flow, which depends on perfusion pressure and on renal vascular resistances: the glomerular filtration rate of single nephrons (SNGFR) also depends on these two parameters. The glomerulus can synthesize all components of the renin-angiotensin system. Renin is produced by mesangial cells and cells of afferent arterioles, angiotensinogen by mesangial cells, and angiotensin I-converting enzyme (ACE) by endothelial and mesangial cells. Ang II binds to afferent and efferent arterioles and to mesangial cells, which subsequently contract and enhance their production of vasodilator prostaglandins. Ang II participates in the regulation of the SNGFR by acting on arteriolar resistance, on the glomerular capillary surface area, and finally on the ultrafiltration coefficient. The synthesis of renin is initiated when the cAMP concentration increases under the influence of hormones (parathyroid hormone, catecholamines). of mediators (e.g., prostanoids), or of changes in composition of tubular fluid (in sodium, chlorine, or calcium) detected by the macula densa. Moreover, renin synthesis is stimulated by decreases in intracellular calcium concentration. The renin-angiotensin system plays an important role in the regulation of glomerular filtration. A decrease in renal plasma flow is followed by an increase in glomerular efferent arteriole resistance, so that perfusion pressure and glomerular filtration remain constant. Furthermore, if the decrease in renal plasma flow is more marked, vasodilator prostaglandins, whose synthesis is stimulated by Ang II, dilate afferent arterioles, which sustains glomerular filtration. Prostaglandins are also apt to bind to the juxtaglomerular apparatus and to increase the intracellular cAMP level. These homeostatic mechanisms can be modified (a) by inhibitors of ACE or of prostaglandin synthesis (nonsteroidal anti-inflammatory drugs), and (b) in several pathologic conditions such as sodium depletion states or vascular nephropathies. In these situations, the glomerular plasma flow and/or glomerular capillary surface area are diminished; it is also possible for renin synthesis and/or for the number or affinity of Ang II receptors to be changed.