Nerve cells with their very long axons and dendrites have an effective system of anterograde and retrograde transport. In the retrograde direction there is, in addition to endogeneous cell constituents, a substantial transport of exogenous material like e.g. trophic factors like NGF. We have found that molecules which bind with high affinity to receptors in the nerve teminal surface membranes (e.g. NGF, tetanus and cholera toxin, lectins) are selectively internalized and transported retrogrdely to the cell body. Although most of them are incorporated into lysosomes in the cell body, they can nevertheless exert specific signal functions in the cell (NGF). Tetanus toxin, however, is released by the dendrites and transferred trans-synaptically to higher order neurons. We suggest that this retrograde and trans-synaptic transport represents an important pathway for "trophic" signal molekules, which convey information from the target cells to the innervating neurons. The same pathways can be used by neurotoxins *tetanus toxin) and neurotrophic viruses for entering the nervous system.