In order to evaluate the effects of age upon fast transport kinetics, we studied the velocity of horseradish peroxidase transport along the optic pathway of neonatal and adult rats. Rate of horseradish peroxidase movement was assessed by following the displacement of horseradish peroxidase activity in the optic nerve with time after injection. We estimated a rate about 100 mm/day for bidirectional transport of horseradish peroxidase in the optic nerve of neonatal rats, while a two-fold higher rate was observed for anterograde transport in adult animals. Developmental regulation of fast transport rate may ensure relative constancy of the time required to connect the cell body with axon terminals.