Nitric oxide (NO), the smallest known bioactive product of mammalian cells, can be produced by most cell types. In the central nervous system NO acts as intracellular and extracellular messenger of a novel generation. NO plays important physiologic role, mediates learning, memory, regulates food intake, drinking and other. Under conditions of excessive formation, NO is emerging as important mediator of neurotoxicity in a variety disorders of the nervous system.