In neonatal mammals, newly grown optic axons are uniformly small in diameter. In the adult, in contrast, axons within the optic nerve can be classified into distinct groups according to their diameter. Because axon diameters are also related to the thickness of the myelin sheath, which in turn determines the velocity of action potential propagation, the question of what determines the axon diameter is of critical importance. In a project aimed at determining the influence of the ensheathing cell on axon maturation, oligodendrocyte development was prevented by eliminating their precursors by unilateral x-irradiation at birth. Axon diameters in both the normal and the myelin-free optic nerves were then measured at varying stages of development. The results demonstrate that axon diameter growth remained substantially reduced in the absence of oligodendrocytes. Interestingly, by x-irradiating the optic nerve and tract on one side of the brain, fibers crossing the chiasm became larger as they went from an unmyelinated nerve to a myelinated tract; fibers on the nonirradiated side became smaller as they went from a myelinated nerve and crossed into the nonmyelinated tract. These results clearly point to a local regulation of axon diameter by oligodendrocytes. Moreover, ganglion cells measured 9 d after the initiation of myelination (postnatal day 6, P6) were of similar size within normal retinas and retinas whose axons were x- irradiated, suggesting that ganglion cell growth occurs in spite of the lack of myelin and axon diameter maturation. Finally, we showed, through both section staining with antibodies to myelin basic protein (MBP) and Northern blot analysis using a probe to MBP, that the x- irradiated nerve began a delayed myelination period (in a gradient from chiasm to eye) at P15 and reached an almost normal myelin pattern at P28. Axons from these nerves grew to seemingly normal diameter concomitant with this delayed myelination.