1. Fast axoplasmic transport in mammalian nerve fibres was determined by the presence of a crest of activity in the sciatic nerve after injection of [(3)H]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 mum in nerve segments taken at the forward part of the crest suggesting that the rate of fast axoplasmic transport is independent of fibre diameter.