1. In rabbit peroneal nerves incubated in vitro at 37 degrees C, acetylcholinesterase (AChE) activity accumulated at both borders of a short region cooled to 5 degrees C. Accumulation was unaffected by concentrations of cycloheximide that inhibited 86% of local protein synthesis, as measured by the incorporation of [3H]leucine. It is probable that the local changes in enzyme activity during incubation reflected redistribution of the enzyme by axonal transport. 2. AChE activity accumulated almost three times faster at the proximal than at the distal border of cooled regions. This suggests that three times more enzyme is normally exported from nerve cell bodies than is returned to them, as though most of the transported AChE were degraded or secreted from distal parts of the neurones. The rates of accumulation of enzyme activity were consistent with average velocities of transport of 24 mm/day in the distal (orthograde) direction and 8.6 mm/day in the proximal (retrograde) direction. 3. When nerves that had been locally cooled for 3 hr were rewarmed to 37 degrees C, the accumulated AChE activity moved rapidly away from the cooled region. More than half of the activity appeared in a wave moving distally with a maximum velocity of 400 +/‐ 35 mm/day. A smaller wave moved proximally with a maximum velocity of 288 mm/day. 4. The observed behaviour of AChE is direct evidence that a small amount of this enzyme, probably less than 10% of the axonal content, is normally transported away from cell bodies as rapidly as any substance known. A still smaller amount of the enzyme is subject to an almost equally rapid retrograde transport. However, 85% of the AChE in peripheral nerve appears to be stationary, which probably explains why the average velocity of transport of this enzyme is so low.
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