We have assessed the NO system in the cardiovascular and renal systems of young, adult and old normotensive (WKY) and hypertensive rats (SHR). The NO pathway was assessed analytically, by measuring the concentration of nitrate in plasma as well as the activity of NO synthases in the left ventricle and kidney; and functionally, by measuring the isometric forces generated upon addition of the NO blocker, L-NAME, to aortic segments. All these procedures consistently revealed that the NO pathway is upregulated in hypertension or senescence. In addition, we have performed immunohistochemical studies of NO synthases in the kidney of adult animals (WKY and SHR). NO synthases are expressed throughout the kidney in both rat strains. Immunoreactivity of neuronal NOS was higher in the tubular cells of the renal medulla of the SHR. Staining with the inducible and endothelial NOS antibodies was similar in normo- and hypertension. In summary, hypertension and ageing upregulate the NO pathway in structures involved in the regulation of blood pressure (heart, vessels and kidney).