In this review we describe what is known about nitric oxide (NO), focusing on its clinical significance. It is now well appreciated that NO is a pivotal endogenous messenger molecule in a variety of physiological and pathophysiological processes. In the cardiovascular system NO participates in the paracrine regulation of vascular tone, body fluid homeostasis and platelet aggregation and adhesion. In the nervous system NO is a neurotransmitter that underpins several functions, including the formation of memory. In addition, NO is produced in large quantities during host defense and immunologic reactions. Perturbation in NO bioactivity has been shown to represent an important pathophysiologic mechanism underlying a number of disease states, such as atherosclerosis, hypertension, diabetes mellitus, and septic shock.