Nitric oxide (NO) has only recently been appreciated as a normal biologic substance with a role in signal transduction. It was first identified as endothelial-derived relaxing factor in blood vessels and as the mediator of the tumoricidal and bactericidal actions of macrophages. NO's role as a neural messenger may be even more prominent. Biosynthesis of NO involves oxidation of the guanidine group of arginine with stoichiometric formation of citrulline. NO synthase is one of the most extensively regulated enzymes in biology. In the periphery, NO is a likely transmitter of nonadrenergic, noncholinergic neurons. In the brain, NO neurons mediate action of glutamate acting at N-methyl-D-aspartate (NMDA) receptors. Excess release of NO appears to account for a major portion of neural damage following vascular stroke.