—Axonal transport was studied in two efferent projections of the substantia nigra: (1) the nigro‐striatal system; and (2) the nigro‐thalamic system. [14C]leucine was injected stereotaxically into the left substantia nigra of rats. At various intervals thereafter significant amounts of [14C]protein were found in the midbrain (which surrounded the injection site), hypothalamus, thalamus and corpus striatum on the injected side of the brain. By determining the temporal characteristics of the distribution of [14C]protein, axonal transport from the substantia nigra to the thalamus and corpus striatum could be inferred. Fast (approximately 50 mm/day) and slow (approximately 1 mm/day) rates of flow were evident in both systems. Either electrolytic lesions of the medial forebrain bundle or intraventricular pretreatment with 6‐hydroxydopamine (200 μg) produced a substantial decrease in the amount of labelled protein reaching the corpus striatum but not in that reaching the thalamus. The decrease following electrolytic lesions correlated significantly with the decrease in the activity of striatal tyrosine hydroxylase, but after 6‐hydroxydopamine the decrease in axonal transport was consistently less than the loss of striatal tyrosine hydroxylase. This difference indicated that a portion (perhaps as much as 20 per cent) of the nigro‐striatal neurons are non‐catecholaminergic. Finally, we present data which suggest that in contrast to effects on peripheral neurons, 6‐hydroxydopamine destroys the cell bodies as well as nerve terminals of adrenergic neurons in the central nervous system.