ABSTRACT
More than 70% of serotonergic neurons have concentrated in raphe nuclei. Raphe nuclei have rested in a rostrocaudal arrangment. They are a neuronal assembling that have joined to gether in midline and appears raphe.To cosideration of important role of serotonin in vital activity, it is necessary that become clear; how is forming the raphe nuclei. Because of resembling between the rat's raphe nuclei and these nuclei in human, the manner of raphe nuclei birth was examined. The 5 embryo of albino rat in 14, 16, 18 and 19 days gestation after sending out of uterus were fixed with boins solution and stained with Kluver and Barrera method. The samples were cutted and evaluated with light microscope. Histological findings showed that in 14[th] day of gestation, two cell groups formed symmetrically; superior [rostral] and inferior [caudal] groups. Rostral cells produce dorsal, median, pontis and magnus raphe nuclei. Caudal cells are the origin of pallidus, obscurus, and magnus raphe nuclei