Monkeys were trained to discriminate with nearly 100% accuracy between a square and a triangle presented simultaneously in a brief tachistoscopic flash. Perceptual masking was demonstrated by inability to perform this trained visual discrimination at better than chance level when the information flash was followed in less than 20 msec by a blank second flash. In order to determine the nature and locus of this retroactive visual perceptual masking effect, electrical potentials were recorded simultaneously from three points along the optic pathways: optic nerve or tract, lateral geniculate body, and visual cortex. Potentials were computer-averaged and correlated with the monkey's ability or inability to make a correct behavioral response to the information contained in the first or test flash (T). When the perception of T was masked by the second or blanking flash (B), only the evoked potential characteristic of B was observed at all recording sites, including the optic nerve or tract. This suggested that the interaction underlying masking occurred in the retina since optic nerve or tract responses are dependent upon retinal ganglion cells. When T was not masked, an early portion of the evoked response to T could be detected at all recording sites. Perception of T was possible when only the initial segment of the T-potential (15 msec or less) was present at each recording site. Thus the visual information transfer essential to the performance of an already learned visual discrimination task appears to occur very early in the course of the evoked response and is not dependent upon later or secondary components.