In previous studies it has been shown that nerve growth factor (NGF) is taken up with high selectivity by adrenergic and sensory nerve terminals and is transported retrogradely to the corresponding cell bodies by a colchicine sensitive mechanism 10,11,23. The present study was designed to investigate whether this retrograde transport of NGF depends on properties of the nerve terminals which are common to all the neurons or restricted to those which respond to NGF either during the whole life cycle (adrenergic neurons) or during a restricted period of embryonic development (sensory neurons). In order to investigate the retrograde transport of NGF in motor neurons we injected [125I]NGF into the musculus deltoideus and measured the side differences of accumulation of radioactivity in the spinal cord (C6-C8) from 4-48 h. At no time was there a preferential accumulation of radioactivity on the injected side. In contrast, tetanus toxin was accumulated preferentially on the injected side and an approximate rate of transport of 7.5 mm/h was calculated. Astonishingly there was also a retrograde transport of tetanus toxin in sensory and adrenergic neurons. The rate of transport was identical to that of NGF and the transport could be blocked by transection of the corresponding nerve fibers and local administration of colchicine. After unilateral injection of [125I]tetanus toxin into the forepaw or the musculus deltoideus light microscopic autoradiographs revealed heavily labeled neuronal cell bodies in the dorsal root ganglia or the ventrolateral spinal cord of the injected side. It is concluded that the retrograde transport of [125I]NGF depends on properties of the neuronal membrane which are specific for adrenergic and sensory neurons, whereas that of tetanus toxin is determined by features which are common to all, or at least to all peripheral, neurons. The fact that the rate of transport for both NGF and tetanus toxin is identical in all examined neurons, supports the hypothesis that the specificity of retrograde transport is determined by specific uptake sites in the neuronal membrane whereas the retrograde transport system is non-specific.