Abstract: The study，using SABC Immune histochemistry technique，was designed for observing the modality，structure and distributed the law of Neuronal Nitric oxide Synthase (nNOS ) positive neurons in rabbit brain. The results showed：①The nNOS positive neurons appeared deep brown which had most in the Cytoplasm and the cell nucleus were more lighter；the neuron cell bodies had all kinds of types，such as triangle，round，ellipse，shuttle and so on；the neuron protruding had one or more；the most positive nerve fibres were brown bunch of pearls kind，some nNOS postive fibres were netted. ② The nNOS positive neurons and nerve fibres were distributed in every area of rabbit brain. And they were widely distributed in cerebral cortex，cerebellum，hypothalamus，mesencephalon and pons. But there were even fewer in medulla . ③Basically，the nNOS positive neurons of the young rabbit had developed to the adult rabbit's level. It developed from maturation to caducity gradually with age. To two years old(aged rabbit), the nNOS positive neurons appeared senescence obviously，including the reducing of density, weakening of expression intensity，decrease of the first born tuber number and shortening of the longest born tuber length etc. The results indicated that nNOS positive neurons and the NO which nNOS positive neurons catalyzed did most use in the nervous system and the nerve adjusting and controlling.

Key words: Rabbit brain, Neuronal Nitric oxide Synthase (nNOS), Positive neurons, Structure, Distributed the law Immune histochemistry histochemical