Understanding of the organization and function of a newly identified neuronal messenger molecule, nitric oxide, has progressed rapidly. Nitric oxide synthase has been purified and molecularly cloned from brain. Its localization is exclusively neuronal and endothelial. The catalytic activity of nitric oxide synthase accounts for the NADPH diaphorase staining of neurons that are uniquely resistant to toxic insults and neurodegenerative disorders. Nitric oxide has diverse functions. In platelets it inhibits their aggregation, in macrophages it mediates cytotoxicity, and in blood vessels it acts as a vasodilator. In the nervous system nitric oxide may be the retrograde transmitter in long-term potentiation. It is the "neurotransmitter" of cerebral vasodilator nerves and the inhibitory "neurotransmitter" of the motor neurons of the intestines. Nitric oxide in situations of excessive production may function as a neurotoxin, suggesting a role for nitric oxide in neurodegenerative disorders.