1. The average velocity of transport of dopamine‐beta‐hydroxylase, an enzyme confined to unmyelinated adrenergic nerves, was measured in rabbit and bullfrog sciatic nerves or branches incubated in vitro at various temperatures. In parallel experiments, the number and density of microtubules were measured in cross‐sections of randomly selected unmyelinated axons from another set of nerves incubated under the same conditions. 2. Average transport velocity was exponentially related to temperature over a wide range, but it fell abruptly towards zero at temperatures below 13 degrees C in the case of rabbit nerves and 10 degrees C in the case of frog nerves. 3. The number of microtubules per unmyelinated axon had declined considerably before the transport of dopamine‐beta‐hydroxylase was impaired. At 13 degrees C, axons in rabbit nerves lost 30% of their microtubules. Axons in bullfrog nerves were, if anything, more sensitive to cold, losing 35% of their microtubules at 15 degrees C and 65% of them at 10 degrees C. 4. Since the temperature‐dependence of the axonal transport of dopamine‐beta‐hydroxylase is probably typical of rapid axonal transport in general, it was concluded that transport in unmyelinated axons can continue unaffected when a substantial fraction of the microtubular population has been lost. 5. Although these results could imply that microtubules are not essential for transport, they are equally compatible with the view that these organelles determine the capacity of the transport system and are normally present in excess.
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