Single-unit recordings were made from striate cortex of kittens in which one eye had been deprived of vision either from birth or subsequently, and for various periods of time. Kittens deprived from birth for 2-3 months showed profoundly defective vision in the deprived eye. In kittens deprived from birth by suturing the lids of one eye or by covering the cornea with a translucent contact occluder, the great majority of cortical cells were actively driven from the normal eye, with normal receptive fields. On the other hand only one cell out of 84 was at all influenced by the deprived eye, and in it the receptive fields in the two eyes were abnormal. In kittens visual experience prior to monocular deprivation reduced the severity of the physiological defects, while three months of deprivation by lid closure in an adult cat produced no detectable physiological abnormality. It is concluded that monocular deprivation in kittens can lead to unresponsiveness of cortical cells to stimulation of the deprived eye, and that the defect is most severe in animals deprived from birth.