Nitric oxide (NO) is a recently discovered and highly unorthodox messenger molecule. Current evidence indicates that, in the CNS, NO is produced enzymatically in postsynaptic structures in response to activation of excitatory amino acid receptors. It then diffuses out to act on neighbouring cellular elements, probably presynaptic nerve endings and astrocyte processes. In several peripheral nerves, and quite possibly in parts of the CNS as well, NO might be formed presynaptically and thus act as a neurotransmitter. In both cases, a major action of NO is to activate soluble guanylate cyclase and so raise cGMP levels in target cells.