The fact that the nucleated center of the nerve cell is the major source of the macromolecular materials required in the maintenance and function of the whole neuron requires the operation of a steady cellulifugal convection of these supplies into and down the nerve fiber. This proximo-distal traffic has been firmly established, but the mechanisms involved in it are still poorly understood. Besides the slow (ca. 1 mm per day) advance of the axonal column as a whole ("axonal flow" in the strict sense), the demonstration of additional, much faster, traffic rates (up to several cm per day) calls for special conduits within the axon ("intra-axonal flow"). To test the possible role of neurotubules (average width:220 A) in this traffic, the drug colchicine, known for its immobilizing effect on microtubules in other types of cells, was locally injected into peripheral nerves. This resulted in a major blockage of the proximo-distal movement of a test enzyme, acetylcholinesterase, into and through the injected zone, the extent of blockage varying with the applied dosage. By analogy, the neurotubules thus seem to be definitely implicated in the motile mechanism of intra-axonal transport. By contrast, the movement of a mitochondrion-associated marker enzyme, diphosphopyridine nucleotide diaphorase, was not perceptibly affected (in the submaximal dosage range), which seems to signify that the proximo-distal shift of mitochondria, for which the slow axonal flow acts as carrier, has gone on uninterruptedly. The experiments thus indicate the possibility of uncoupling the axonal and intra-axonal transport mechanisms.