To account for the transport in nerve fibers of tubulin and neurofilament proteins in slow component a, the Structural Hypothesis holds that these proteins are assembled into microtubules and neurofilaments in the cell bodies and the cytoskeletal organelles then moved down in the fibers as part of an interconnected matrix at a uniform rate of about 1 mm/day. The Unitary Hypothesis, on the other hand, considers these proteins to be carried down within the fibers as soluble components or as freely movable small polymers or subunits turning over locally in the stationary cytoskeleton. To differentiate between the two hypotheses, cat L7 dorsal roots were taken at times from 7 to 25 days after their L7 dorsal root ganglia were injected with [3H]leucine to assess the labeling of the cytoskeleton by the use of beading and autoradiography. Beading was induced by a mild stretch and after fast-freezing and freeze-substitution of the roots for histological preparation, the beads were seen in the fibers as a series of expanded regions alternating with constrictions. In the constrictions the cytoskeleton was compacted into an area as small as 5% that of the normal axon, with the axoplasmic fluid and displaceable (freely movable) components squeezed from the constrictions into the adjoining expansions. Roots taken after 7 and 14 days, times consistent with slow component a downflow, were assessed with sodium dodecyl sulfate-polyacrylamide gel electrophoresis and their content of tubulin and neurofilament proteins shown to constitute 40-50% of all the labeled proteins present. In autoradiographs of dorsal roots taken at those times, numerous grains due to radioactivity were located over the non-constricted regions of the fibers. Few or no grains were present over the constrictions after 7 days. The findings are in accord with the labeled tubulins and neurofilament proteins being present in soluble form in the fibers and expressed from the constrictions into the expansions of the beaded fibers. In contrast, a number of fibers in roots taken at 14-20 days after injection showed somewhat higher grain densities over the constrictions, and more so after 25 days, indicating uptake of labeled subunits into the cytoskeletal organelles at later times. The results are consistent with the downflow of tubulin and neurofilament proteins as soluble components which drop off in the axon to turn over locally in their respective cytoskeletal organelles.