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1. The giant cerebral neurone of the sea hare, Aplysia californica, is a unipolar serotonergic cell. Its axon bifurcates, one branch travelling in the cerebrobuccal connective, the other in the posterior lip nerve. 2. I13H]serotonin was injected under pressure into the cell body of the giant cerebral neurone. We studied fast axonal transport of the radioactive transmitter substance along the lip nerve when the cerebrobuccal connective was cut close to the bifurcation. 3. When the connective was cut, more than twice as much [3H]serotonin was transported along the lip nerve compared to uncut control nervous systems. 4. The increased [3H]serotonin appearing in the nerve probably was originally destined to enter the connective, but was diverted from the cut stump which was occluded with backed-up material. 5. The incremental [3H]serotonin in the lip nerve was not the result of increased export from the soma in response to injury. 6. Not only was more [3H]serotonin transported along the lip nerve, but also a far greater fraction of the transmitter moved at very fast transport rates, approaching 120 mm/day. In uncut control nerves only a small fraction of total [3H]serotin moved faster than 70 mm/day. 7. These results are interpreted with a model for fast axonal transport. We suggest that serotonergic vesicles move at a fixed, maximal speed when attached to essentially immobile tracks, but that the vesicles are only intermittently associated with the tracks. We presume that the rate-limiting step in movement of vesicles is the concentration-dependent and reversible binding to the tracks. Transport along axons may be considered analogous to those enzymatic reactions in which formation of the enzyme-substrate complex limits the appearance of product. Translocation is here analogous to formation of product. The process may therefore be approached theoretically by modification of the Michaelis-Menten formulation.