Nitric oxide(NO) is thought to play an important role in development and plasticity of brain. In this study, we aimed to examine the expression of neuronal NOS and NADPH-diaphorase (NADPH-d) activity in the developing rat brain. The results show that there is a great variation in the time of appearance of the earliest NOS containing cells depending on their location: At the 15th embryonic day weakly stained cells were present in caudate-putamen, and neurons in the sensory trigeminal nucleus and the solitary nucleus displayed an intense staining. The NOS neurons in orbital neocortex, bed nucleus of stria terminalis, paraventricular hypothalamic nucleus, lateral hypothalamic area and mammillary body appeared first at the 18th embryonic day. The supraoptic nucleus and superior and inferior colliculi also weakly labeled at the 18th embryonic day, At the loth embryonic day, positive cells appeared in horizontal limb of diagonal band, anterior olfactory nucleus and parafascicular thalamic nucleus. In the cerebellum, weak NOS staining was present in fibers and cells situated below Purkinje cert layer. The Purkinje cell layer displayed a weak, rather diffuse activity throughout the cerebellum at postnatal day 0. At the 4th postnatal day. the reaction product in the Purkinje cell layer became more distinct. At the 10th postnatal day, the inner part of molecular layer became populated by NOS positive basket cells, and the reaction products on the Purkinje cells began to disappear. The present results showed that NOS in the rat brain is expressed in different populations of neurons at different stages of development. This expression pattern of NOS suggests that NO may play a role in the developmental remodelling of the mammalian brain.