The effects of dark rearing on the time course of the postnatal critical period for monocular deprivation (MD) in visual cortex were determined in cats who experienced 2 days of MD at various postnatal ages. In normal development, plasticity (susceptibility to MD) was very low at 3 weeks, rose sharply at 6 weeks, and gradually declined over the next 10 weeks. This developmental profile was dramatically altered by dark rearing which slowed the entire time course of the critical period. In dark reared cats, plasticity rose steadily over the first 12 weeks, and was maintained at 16 weeks. There was a crossover of the two profiles of the critical period such that at young ages (6 weeks) normal cats were more plastic than dark reared cats while at later ages (after 9 weeks) dark reared cats were more plastic. A second experiment indicated that dark rearing slowed down the progression of the critical period even after it had been initiated by a period of normal vision. MD produced substantial effects after the normal critical period in cats who were reared normally for the first 6-8 weeks of life and then placed in darkness until 5 months of age. The results are discussed in terms of a simple model of the accelerating effect of visual input and the decelerating effect of total darkness on the time course of the critical period.