When pyramidal tract axons are cut in the adult hamster, fibers degenerate in both anterograde and retrograde directions from the lesion. If the same operation is performed on infant hamsters, however, there is massive regrowth of the severed axons via a new brainstem pathway to their appropriate terminal sites in the medulla and spinal cord. In contrast to previous studies, these results suggest that axons in the mammalian central nervous system damaged early in life may regenerate in a functionally useful way.