Catecholaminergic systems were visualized in the CNS of human embryos from stage 15-16 (5 gestational week, g.w.) to 18 (6 g.w.) using tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH) as immunocytochemical markers. At 5 g.w., several TH-like immunoreactive (TH-IR) cell groups were identified in the medulla oblongata, pons, mesencephalon and the anlage of the hypothalamic area. DBH immunoreactivity was restricted to the locus coeruleus and to rare neurons in the medulla oblongata. At 6 g.w., the density of TH-IR neurons was strikingly increased in these different areas--especially in the prospective substantia nigra and ventral tegmental area--and two main bundles of catecholaminergic axons extended from the medulla oblongata until the basal forebrain and from the mesencephalic tegmentum to the anlage of the striatum. These pathways were mainly TH-IR but DBH-IR was also observed in the former. No TH-IR fibers reached the telencephalon at 6 g.w.