Nitric oxide (NO) is synthesized by at least three distinct isoforms of NO synthase (NOS). Their substrate and cofactor requirements are very similar. All three isoforms have some implications, physiological or pathophysiological, in the cardiovascular system. The endothelial NOS III is physiologically important for vascular homeostasis, keeping the vasculature dilated, protecting the intima from platelet aggregates and leukocyte adhesion, and preventing smooth muscle proliferation. Central and peripheral neuronal NOS I may also contribute to blood pressure regulation. Vascular disease associated with hypercholesterolaemia, diabetes, and hypertension is characterized by endothelial dysfunction and reduced endothelium‐mediated vasodilation. Oxidative stress and the inactivation of NO by superoxide anions play an important role in these disease states. Supplementation of the NOS substrate L‐arginine can improve endothelial dysfunction in animals and man. Also, the addition of the NOS cofactor (6R)‐5,6,7,8‐tetrahydrobiopterin improves endothelium‐mediated vasodilation in certain disease states. In cerebrovascular stroke, neuronal NOS I and cytokine‐inducible NOS II play a key role in neurodegeneration, whereas endothelial NOS III is important for maintaining cerebral blood flow and preventing neuronal injury. In sepsis, NOS II is induced in the vascular wall by bacterial endotoxin and/or cytokines. NOS II produces large amounts of NO, which is an important mediator of endotoxin‐induced arteriolar vasodilatation, hypotension, and shock. Copyright © 2000 John Wiley & Sons, Ltd.