Previous studies have shown that the response properties of cortical cells of cats reared in darkness throughout the naturally-occurring critical period can still be modified by environmental manipulation. Here the effects of extremely prolonged dark-rearing, and of light exposure before or after dark-rearing on subsequent susceptibility to monocular deprivation are examined. Monocular deprivation remains effective in altering cortical ocular dominance even in cats which have been dark-reared for up to 2 years, long after the end of the naturally occurring critical period. The effects of monocular deprivation are, however, less pronounced in these cats than in animals dark-reared for shorter periods of time. The effects of visual exposure before dark-rearing were examined by allowing kittens 2 months of normal vision followed by 3 months of dark-rearing. Then, the effects of monocular suture were compared among these kittens and: (1) 2-month-old normal kittens (which had the same amount of visual exposure as the experimental animals); and (2) 5-month-old normal kittens (of the same age as the experimental animals). The results show that monocular deprivation is more effective in the experimental animals than in normal kittens of the same age, but less effective than in normal 2-month-old animals. The effects of visual exposure after dark-rearing were examined by allowing animals dark-reared for 4 months varied durations of binocular visual exposure before monocular suture. Susceptibility to monocular suture disappears about 6 weeks after the dark-reared animals are brought into the light.