The development of a simple and “gentle” technique for the radio-iodination of proteins with lactoperoxidase (1) and subsequent modifications of this technique (2–6) have made it possible to introduce labeled neurotrophins as sensitive tracers into living organisms and to determine their fate. Radiolabeled neurotrophins have been used to examine receptor-mediated internalization, transport, and metabolism, both in vivo (7–10) and in vitro (11,12). They have been instrumental in the analysis of functional interactions of neurotrophins and their receptors (13) and the discovery of retrograde axonal transport (7,8), a cornerstone of the neurotrophic hypothesis (14), as well as the discovery of anterograde axonal transport of neurotrophins (15). The neurotrophin dependence of clinically important neuronal populations has been revealed by transport studies with radiolabeled neurotrophins (16,17).