While there can be substantial recovery of vision during reverse occlusion in the deprived eye of kittens that have been monocularly deprived from birth, in many situations this visual improvement does not appear to be retained following the introduction of binocular visual input. This study examines the consequences of periods of reverse occlusion whose onset and duration would be thought to maximize the opportunity for good and permanent recovery of vision in the initially deprived eye. Twenty kittens were monocularly deprived from near birth to either 4, 5, or 6 weeks of age and then reverse occluded for periods that were either very brief (9-18 d) or very long (9-12 weeks). In the former situation, binocular visual experience was introduced when the visual cortex was most susceptible to environmental modification, which would be thought to maximize the opportunity for the eventual development of good vision in both eyes. On the other hand, the long periods of reverse occlusion might be expected to lead to a permanent recovery of vision in only the initially deprived eye, since binocular visual input would have been introduced at a time when the cortex was no longer very plastic. Surprisingly, the end result of both these extreme regimens of reverse occlusion was the same, namely, a severe bilateral amblyopia in which the acuity of both eyes was permanently reduced to levels that were as low as 1/10, but more typically between 1/3 and 1/4 of normal values. Contrast sensitivity functions measured on 2 of the cats that received a brief period of reverse occlusion revealed deficits in contrast sensitivity of about an order of magnitude at all spatial frequencies. These observations have important implications for the nature of the physiological and anatomical changes that occur in the visual cortex during reverse occlusion.