Tyrosine hydroxylase (TH) immunoreactive (IR) central catecholaminergic neurons have been observed in human CNS from 4.5 gestational weeks (g.w.) on [Verney, C., Zecevic, N. and Puelles, L. Eur. J. Neurosci., Suppl. 8 (1995) 7044; Zecevic, N. and Verney, C., J. Comp, Neurol., 351 (1995) 509-535]. We describe here a discrete TH-IR cell population localized in the rostral nasal region during embryonic development. Tyrosine hydroxylase-IR cells spread from the olfactory placode towards the basal and medial telencephalon. They follow the same migration path as the gonadotropin-releasing hormone (GnRH)-IR hypothalamic neurons. Tyrosine hydroxylase-IR neurons are first detected at 4.5 g.w., while GnRH-IR cells are visualized later at 5.5 g.w. Double immunocytochemical labeling reveals the presence of three neuronal populations comigrating along the developing vomeronasal-nervus terminalis complex. These populations express either one or both TH and GnRH phenotypes depending on their position in the migration route. At 6 g.w., most of the neurons express TH immunoreactivity as they leave the vomeronasal organ whereas most of the GnRH-IR neurons are detected closer to the CNS and in the CNS itself. These results emphasize the early phenotypic heterogeneity of the different migrating neuronal populations generated in the olfactory placode in humans. At later stages, very few TH-IR neurons are detected in the anterior forebrain suggesting a transient expression of TH immunoreactivity within these neuronal populations.