Nitric oxide (NO) is a gas that is involved in many biologic processes. It is synthesized, by the NO synthases. The synthesis of NO by vascular endothelium is responsible for the vasodilator tone. In the central nervous system, it is a neurotransmitter, involved in the formation of memory; in the periphery, some nerves operate through NO-dependent mechanisms to mediate neurogenic vasodilatation in the respiratory, gastrointestinal, and genitourinary tracts. NO also contributes to control of platelet aggregation and regulation of cardiac contractility. In addition, NO is produced in large quantities during host defense and immunologic reactions. Because it has cytotoxic properties and is generated by activated macrophages, it is likely to have a role in nonspecific immunity. This review discusses the physiology, the mechanism of action, and the role of NO in inflammation and immune responses.