A sensitive period exists in humans and experimental animals during which certain visual functions are modifiable by decreased or abnormal visual stimulation. Experiments in monkeys have shown that, at the beginning of this sensitive period, the manifestations of the visual deprivation syndrome develop more rapidly at the cortical than at the geniculate level. Moreover, a shift of cortical dominance as the result of reverse suturing may occur while the geniculate nucleus still reflects the effects of the original deprivation experiment. At the end of the sensitive period the behavioural, electrophysiological, and histological aspects of the visual deprivation syndrome become less predictable. Cortical physiology remains sensitive to abnormal visual input beyond the first 3 months of life. Clinical examples and animal experiments also suggest that the reversal of the effects of visual deprivation may occur long after maturation of the visual system is completed. These findings support the view that abnormal visual stimulation during visual infancy causes a functional reorganization rather than an arrest of development of anatomical retinocortical connections.