The number of axons in the optic nerve of the newborn rat has been compared with the number present in the adult animal. Nerves taken from animals on the day of birth contain 242,000 +/- 29,000 (S.D.) fibres (n = 5). By the sixth postnatal day, the number of axons has fallen to the stable values of adults (99,000 +/- 3700, n = 8). Thus development of the rat's visual system during the first 5 days of life is associated with a loss of 60% of the axons present in the optic nerve at birth. Counts made on the remaining nerve after enucleation of one eye suggest that the presence of retino-retinal axons during the first 5 postnatal days cannot account for all of this reduction.