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.