NO is a free radical that regulates a variety of developmental modulation processes. NO is synthesized by NOS and it acts as a neurotransmitter, neuromodulator or second messenger molecule in the central nervous system. Since NO production may be different before or after birth, the expression of neuronal nitric oxide synthase was examined and analysized during the development of rat optic vesicle from embryonic day E14 to E18 by histochemical procedures. The samples were frozen and cut on a cryostat and then studied by using the light microscope. Expression of nNOS was first seen on E14 in cells of Cajal-Retzius located in the marginal zone of optic vesicle. NADPH-d persisted in this layer throughout the embryonic period and began to decrease on E20. On E16, the optic vesicle had four NADPH-d positive layers. On E18,NADPH-d reactivity observed at low magnification was clearly defined in the fifth layer. In the late stages, the most notable feature was a decrease in histochemical reaction of the marginal zone, i.e. the fourth layer showed less staining than the rest of the cortical plate. The observations suggested that nitric oxide is synthesized during embryonic life processes and it changes with ageing in the rates of NOS-reactive cells in embryonic life and maturation processes. These results indicate that NOS makes different contributions in the optic vesicle NO production along with ageing.