Nitric oxide is a ubiquitous and unique biological messenger molecule. It mediates blood vessel relaxation by endothelium, immune function of macrophages, and neurotransmission of central and peripheral nervous systems. Endothelial and neuronal nitric oxide synthases are constitutively expressed and activated by calcium entry into cells, whereas the macrophage nitric oxide synthase is inducible with new RNA and protein synthesis upon immune stimulation. Nitric oxide may play a role in the neurotransmitter release, neural development, synaptic plasticity, and regulation of gene expression. Excessive production of nitric oxide is neurotoxic and is implicated in a variety of neurological disorders.