Electroencephalograms were obtained from cats which had been raised in continuous darkness from before eye opening to approximately 1 year of age, and compared to those obtained from a control group reared in the lighted laboratory. Rearing in darkness produced no detectable electrocortical abnormalities in the “spontaneous” record; however, when the response from the visual cortex of dark-reared animals to intermittent photic stimulation was investigated it was found that the flicker-induced potentials were reduced at all frequencies of stimulation utilized (1–50 flash/sec). All light-reared and no dark-reared animals demonstrated visually-mediated responses as judged by performance on five simple behavioral tests. When two dark-reared animals were placed in the experimenter's home their scores on all five behavioral tests changed from negative to positive within 10 days. Two additional dark-reared animals placed in a cage in the lighted laboratory required over 30 days of experience before positive scores were obtained on these tests. This research demonstrated that the behavioral deficits produced by the exclusion of the adequate stimulus from the retina during the postnatal developmental period does not necessarily produce electrophysiological abnormalities in the visual cortex and that these profound behavioral deficits may be very rapidly altered when the proper conditions are utilized.