Nitric oxide is generated from the terminal guanidino nitrogen of L-arginine yielding citrulline. This reaction is catalyzed by two major types of nitric oxide synthase: inducible and constitutive. Nitric oxide is a gaseous mediator responsible for a variety of physiologic phenomena. Its short half-life in biologic systems has created problems in its direct determination. Many experiments depend on the use of inhibitors of nitric oxide synthase to provide indirect evidence for the involvement of nitric oxide. Spectroscopic and electrochemical methods are the best for the direct measurement of nitric oxide; however, the advantages and disadvantages of each technique should be considered carefully before a specific method is selected.