Catecholamine-containing neuronal processes penetrate to the outer superficial layers of the developing neocortex via the lateral neocortex anlage by embryonic day (ED) 16, only 48 hours after the final cell division of the catecholamine neurons in the mesencephalon and pons-medulla. Sagittal sections were taken from a series of perfused embryos at precise time-intervals after insemination. The corpus striatum receives a large catecholamine input and serves as a reference for tracing the very fine varicose processes that course through the caudate nucleus to innervate the more rostral structures of the developing neocortex cerebri. The input fibers to the neocortex arrive via three to four small fiber bundles, entering chiefly at the ventro-rostral aspect. The bundles than bifurcate into the deep and superficial layers of the cortex. Between ED16 to ED21 the innervation progresses in ventral to dorsal and rostral to caudal directions. Embryonically, fluorescent fibers are observed in the outermost superficial layer and in the intermediate zone, below the cortical plate; only rarely are they seen crossing the cortical plate. The demonstration of monoaminergic neuronal fibers reaching neocortical structures by ED16 adds further weight to the speculation that they may play a role in induction and differentiation, and suggests that post-natal experimental manipulations using ascending-bundle lesions will have been performed at least five days after the arrival of catecholamine fibers at their cortical destinations.