The duration of the sensitive period of the kitten visual cortex to the effects of monocular deprivation was explored by studies of the behavioral and physiological recovery from extended periods of monocular occlusion imposed from birth, and by examination of the physiological effects of a 3 month period of monocular occlusion imposed on animals at either 4, 5, 6, 7 or 8 months of age. Animals monocularly deprived until 4 months of age eventually show considerable behavioral and physiological recovery from the severe deficits observed immediately following termination of the period of deprivation. The conclusion that binocular connectivity may still be altered by the nature of the animal's visual input beyond 4 months of age was supported by the results obtained from animals that were monocularly deprived at 4 months of age or older. Animals deprived at either 4, 5 or 6 months showed a clear shift of cortical ocular dominance in favour of the non-deprived eye, but those deprived at 7 or 8 months showed approximately normal ocular dominance distributions. It is concluded that the sensitive period lasts at least twice as long as previously thought, to between 6 and 8 months of age.