We have compared the polypeptides undergoing axonal transport in the retinal ganglion cells of neonatal and adult rabbits, and have observed the following: (1) Representative polypeptides of five different adult transport groups are axonally transported from the time of birth. (2) Polypeptides of group IV (a group that includes actin and myosin) are transported two-fold more rapidly in neonates than in adults. (3) Two polypeptides, M (145K) and L (73K) that are components of neurofilaments and move with the fifth, slowest group of transported proteins, are transported approximately eight-fold more rapidly in neonatal rabbits than in adults. (4) H, a third group V polypeptide, that serves to crosslink neurofilaments, was not detected in the rabbit optic nerve until 12 days after birth. We consider the possibility that the late induction of the crosslinker precipitates a cytoskeletal "phase transition" that may be responsible for the developmental alterations in apparent transport velocities, and may have additional consequences for neuronal development.