Following initial rearing in either total darkness or normal illumination, kittens at different ages were subjected to right-eye closure and various periods of vision through the left eye. After the period of monocular vision, single units in striate cortex were tested for visual responsiveness through each eye. A severe reduction in the proportion of units responsive to the deprived eye occurred over the first few days of monocular vision. Functional abnormalities were variably present after 1 day, marked after 2.5 and 3.5 days, and complete after 10 days. Monocular vision produced very much the same effect on ocular dominance of striate units, provided age and duration of suture were identical, regardless of whether kittens had received prior dark- or light-rearing.