It is now well established that the anatomical and functional development of the central visual pathways of a number of higher mammalian species is activity-dependent [1-3]. This dependence was revealed by the functional effects of an early period of monocular deprivation, where one eye of a young animal was deprived for a time of patterned visual input. Subsequently, most cells in the visual cortex (area 17) could be excited only by visual stimuli delivered to the non deprived eye [4-6] and the animal appeared blind through the deprived eye [7,8]. These effects have been attributed to a competitive activity-dependent mechanism in development, whereby the two eyes compete for control of cortical cells [9,10]. There are, however, suggestions that the substantial recovery that can occur after monocular deprivation may be mediated by a different mechanism. Here, insight into the nature of this mechanism has been provided by monitoring the speed of changes in the vision of the deprived eye of a kitten after 6 days of monocular deprivation. Although both eyes were open during the recovery period, the kitten was able to see with its deprived eye only 2 hours after visual input was restored to this eye. The visual acuity of this eye improved rapidly in the first 24 hours and continued in an orderly way for 6 weeks. In contrast to the effects during monocular deprivation, which depend upon a competitive activity-dependent process, we propose that the events that follow deprivation rely on a mechanism driven by the absolute level of visually evoked activity through the formerly deprived eye.