The principal challenge in the research of radical biology lies in developing a solid causal relationship between the tissue productions of various reactive species, long recognized to have potent and toxic target molecule reactions, and their contribution to cell or tissue dysfunction. Not until this is accomplished can a rational therapeutic strategy for oxidant tissue injury be devised. These dilemmas amplify the immense challenge that is faced upon in development of a clear understanding of the multifaceted role that nitric oxide (NO) plays in vascular disease. The high rate of production and broad distribution of sites of production of NO, combined with its facile direct and indirect reactions with metalloproteins, thiols, and various oxygen radical species ensure that NO plays a central role in regulating vascular physiological and cellular homeostasis as well as critical intravascular free radical and oxidant reactions. This concept is emphasized in this chapter, using atherosclerosis as a prime example of the central role that reactive species play in vascular diseases. The chapter describes how superoxide anion (O2−) “inactivates” the vasorelaxant actions of NO in atherosclerotic vessels, leading to impaired endothelial cell (EC)-dependent relaxation and a propensity for vasospasm. The alterations in vascular reactivity associated with atherosclerosis are related to changes in EC-dependent mechanisms of relaxation. Acetylcholine and other EC agonists normally promote the relaxation of isolated vascular ring segments by stimulating the production of NO. Nitric oxide diffuses to underlying vascular smooth muscle cells, where it activates soluble guanylate cyclase and induces vessel relaxation via cGMP-dependent mechanisms. © 1995, Academic Press, Inc.