In adult fowls the time-course of the neurotoxic effects of 5,6-DHT on indolaminergic neurons (in comparison with controls and with animals pre-treated with mebanazine, a MAO-inhibitor) was studied. It was shown that 5,6-DHT given into the third cerebral ventricle produced a rather selective degeneration of indolaminergic neurons which was most dramatic 4 and 7 days after the injection. Injured axons characterized by a pile-up of strongly yellow fluorescent material were seen in areas bordering the third ventricle, and in the posterior, medium and anterior hypothalamus, preoptic area, median eminence, n. tuberalis and paleostriatum augmentatum. A decrease in yellow fluorescence in areas distal to the injured axons was only detectable after 19 days. A certain regrowth of some terminal networks was observed 30 days after 5,6-DHT administration. Present data indicate that, in fowls, ascending indolamine neurons mainly originate in tegmental cell bodies and are organized in a large tract which runs along the medial forebrain bundle, sending terminals innervating several diencephalic and other subcortical areas.