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SUMMARY1. Fast axoplasmic transport in mammalian nerve fibres was determined by the presence of a crest of activity in the sciatic nerve after injection of [3H]leucine into the L7 dorsal root ganglion or the L7 motoneurone region in the ventral horn region of the spinal cord. After incorporation into proteins by the cell bodies, a rate of transport close to 410 mm/day was found for cat sensory nerves. A closely similar rate was found in the motor and sensory sciatic nerve fibres of the monkey, dog, rabbit, goat and rat. In the longer nerves where longer downflow times were possible, there was no decrement of rate with distance, or presence of later appearing crests of activity indicative of multiple fast transport systems.2. The rate of fast transport found in the long L7 dorsal roots of the rhesus monkey was the same as that in the corresponding length of sciatic nerve and the same fast rate was shown by the crest of activity ascending in the dorsal columns of the spinal cord.3. Labelled activity was found present inside myelinated nerve fibres ranging in diameter from 3 to 23 ,tm in nerve segments taken at the forward part of the crest suggesting that the rate of fast axoplasmic transport is independent of fibre diameter.

Acetylcholinesterase (acetylcholine acetyl-hydrolase, EC 3.1.1.7) is carried down mammalian nerve fibres by the fast axoplasmic transport system. This conclusion was derived from experiments involving the ligation of cat sciatic nerves at two sites placed 8 3 3 mm apart. The enzyme accumulated in segments of nerve proximal to the upper ligation in a linear fashion over a period of at least 20 h. At approximately 5 h the accumulation of enzyme ceased in the nerve segment proximal to the distal ligation within the isolated length of nerve, an observation indicating that the portion of AChE free to move within the isolated nerve had been depleted during this period of time. The freely moving fraction of AChE was estimated to be 15 % of the total enzyme activity present in the nerve (10% in the proximo-distal direction and 5 % in the retrograde direction). The rate of AChE downflow (as estimated from the intercept of the curve plotting accumulation with the line denoting when depletion started) was 431 mm/day within a 95% confidence interval of 357-543 mm/day. In view of the variability, our results demonstrated that AChE was being carried by the fast axoplasmic transport system, which in earlier studies was estimated to have a characteristic rate close to 410 mm/day. An accumulation of AChE was also found on the distal side of the ligations that represented a movement of AChE in the distal-proximal direction in the fibres. This retrograde transport was smaller in amount (about one-half) than the proximo-distal rate of transport, or close to 220 mm/day. The rate of AChE transport was discussed in relation to the 'transport filament' hypothesis of fast axoplasmic transport.

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