Abstract Expression of NADPH-diaphorase (NADPH-d) was studied in the rat telencephalon and diencephalon from embryonic day 15 (E15) to postnatal day 30 (P30). The study has focused on the first appearance of NADPH-d staining in some areas which show high expression during adult life. The time of appearance ranged from E15 to the first days following birth, depending on the location of the cells. In many regions, neuronal processes, when staining appeared, were observed in close relationship with cerebral vessels. A possible role for nitric oxide in brain development should be explored.