Segregation and stabilization of thalamocortical afferents to eye-specific patches, so-called "ocular dominance (OD) columns," in visual cortex are hypothesized to be based on activity-dependent competition for trophic factors such as brain-derived neurotrophic factor (BDNF) between afferents representing the two eyes during the critical period of postnatal development. To test this hypothesis we observed effects of an intracortical infusion of BDNF on OD columns in monocularly deprived kittens and also compared effects between normal kittens and adult cats. BDNF had a hypertrophic action on afferents irrespective of visual inputs so that it desegregated OD columns in the visual cortex of deprived and normal kittens, but this action was not seen in the adults, substantiating its hypothesized trophic role in plasticity of OD columns in the developing visual cortex.