The formation of nitric oxide (NO) from the amino acid L-arginine is now recognized as a ubiquitous pathway involved in a number of physiological functions. The generation of NO by the vascular endothelium maintains a vasodilator tone that is essential for the regulation of blood flow and pressure. Nitric oxide also contributes to the control of platelet aggregation and the regulation of cardiac contractility. In the brain, NO acts as a mediator of cell-cell signalling. In the peripheral nervous system the release of NO from many nerves previously classified as non-adrenergic and non-cholinergic maintains various gastrointestinal, respiratory, and genitourinary tract functions. These properties of NO are all mediated by the action of constitutive NO synthases and subsequent activation by NO of the soluble guanylate cyclase. In addition, NO is produced in large quantities by an inducible NO synthase (iNOS) during host defense and immunological reactions. Because it has cytotoxic properties and is generated by activated macrophages, NO is likely to have a role in non-specific immunity. Thus, this deceptively simple gaseous molecule performs a wide variety of physiological functions. This brief review will focus on the roles played by NO in the physiology and pathophysiology of the cardiovascular system.