Subjected 70 immature monkeys trained on 3 separate visual discrimination problems to bilateral lesions of the inferotemporal cortex, the posterior thalamus, the hypothalamus, or the midbrain. After a postoperative recovery period Ss were tested for retention. Those lesions which consistently led to severe losses in retention were located either within the inferotemporal cortex, the pulvinar, or the pretectal-nucleus posterior thalami region. Lesions of the midbrain (superior colliculus, subcollicular area, central gray substance, reticular formation, red nucleus, substantia nigra, or central tegmentum) produced mixed results. Findings are compared with those obtained using the rat and cat and are interpreted in terms of a corticofugal system which may be posited to mediate visual discrimination performance. (52 ref.) (PsycINFO Database Record (c) 2006 APA, all rights reserved)