Nitric oxide( NO) is a ubiquitous and unique biological messenger molecule and effector.Discoveries have been made regarding the physiologic and pathologic roles of NO in the nervous system.Effects of NO have been identified on neurotrans-mitter release,neural morphogenesis,regulation of gene expression and cerebral blood flow.NO may play a role in long-term potentiation of synaptic transmission in the hippocampus and long-term depression of synaptic transmission in the cerebellum as a retrograde messenger.Excessive production of No is neurotoxic and is implicated in a variety of neurological disorders.