Recent studies (Cynader and Mitchell, '80; Mower et al., '81) have shown that total dark rearing prolongs susceptibility to the physiological effects of monocular deprivation (MD) in visual cortex beyond the normal age limits. The present study addressed whether this delayed physiological plasticity is accompanied by delayed anatomical plasticity in the geniculocortical pathway. Ocular dominance (OD) columns as defined by transsynaptic autoradiography following injection of 3H proline into one eye were studied both qualitatively and quantitatively in 17 cats. Compared to normal rearing (N-3), both binocular eyelid suture (N-2) and total dark rearing (N-3) resulted in incomplete segregation of OD columns in area 17. This apparent immaturity after binocular deprivation, however, did not reflect a delayed capacity for development and plasticity. Visual experience after dark rearing produced no marked changes. In cats who experienced MD after dark rearing, injection of either the nondeprived (N-2) or deprived eye (N-3) resulted in a nearly uniform distribution of label throughout layer IV of area 17. The same result occurred with binocular vision after dark rearing (N-1). MD from birth, however, produced expansion of columns from the nondeprived eye (N-1) and contraction of columns from the deprived eye (N-1). MD imposed after 4 months of normal vision resulted in normal OD columns (N-1). Electrophysiological studies revealed a high proportion of binocular cells within layer IV in cats who experienced monocular or binocular vision after dark rearing. Outside of layer IV there were clear environmental effects on OD of single cells in these cats. Measurements of cell sizes in the clateral geniculate nucleus showed shrinkage of cells innervated by the deprived eye when MD was initiated at birth (N-3). MD after dark rearing (N-4) produced no differences in cell sizes. It is concluded that visual input is necessary for the formation of normal OD columns, the critical period for formation and environmental modification of OD columns is limited to early life, and the physiological effects of visual experience after dark rearing reflect changes occurring beyond the geniculocortical pathway.