Male rats were placed in complete darkness from birth until 30 days of age, followed in some cases by a 35 days period of rehabilitation in control lighting conditions. Groups of control and experimental animals were killed at 30 and 65 days of age by perfusion with buffered 2.5% glutaraldehyde. The right optic nerve was dissected out from each animal and processed for embedding in Epon. Quantitative stereological procedures were used to estimate the total number of both myelinated and non-myelinated optic nerve fibres and their mean minimum diameters. There were no significant differences in the total number of optic nerve fibres between dark- and light-reared rats. However dark-reared rats had myelinated and non-myelinated fibres with significantly larger fibre diameters than those in age-matched light-reared rats. The proportion of optic nerve fibres which were myelinated increased with age in both groups of animals. However by 65 days of age the degree of myelination was slightly but significantly greater in the previously dark-reared rats than in the light-reared controls. These results indicate that rats reared in complete darkness for the first 30 days of postnatal life show morphological changes in the optic nerves. The possible significance of these changes is discussed.