Hypothalamic gonadotropin-releasing hormone (GnRH) neurons originate in the olfactory placode and migrate to the forebrain during embryonic development. We found that GnRH neurons migrated in two different modes in the chick medial telencephalon: they initially underwent axophilic migration in association with a subset of olfactory fibers in a dorsocaudal direction. This was followed by ventrally directed tangential migration to the basal forebrain. Since many of the ventrally migrating GnRH neurons did not follow distinct fiber fascicles, it is proposed that diffusible guidance molecules played a role in this migratory process. A long-range diffusible factor, netrin 1, was expressed in the lower part of the commissural plate and the subpallial septum, but not along the axophilic migratory route of GnRH neurons. Failure of ventrally directed migration of GnRH neurons and their misrouting to the dorsomedial forebrain was induced by misexpression of netrin 1 in the dorsocaudal part of the septum near the top of the commissural plate, which is where the migration of GnRH neurons changed to a ventral direction. In such cases, a subset of olfactory fibers also extended, but close contact between aberrant fibers and misrouted GnRH neurons did not exist. A coculture experiment demonstrated that netrin 1 exerts an attractive effect on migrating GnRH neurons. These results provide evidence that netrin 1 acts as chemoattractant to migrating GnRH neurons at the dorsocaudal part of the septum and has the potential to regulate the ventral migration of GnRH neurons to the ventral septum and the preoptic area. J. Comp. Neurol. 518:2019-2034, 2010. (C) 2010 Wiley-Liss, Inc.