Examined the effects of brief periods of eye closure on the visual system of kittens in Ss monocularly deprived by eyelid suture for between 6 hrs and 8 days, starting on the 29th day of life. Before that time, the kittens had normal visual experience. Perceptible shifts in cortical binocularity were seen following as little as 1 day of eye closure, and these effects were increasingly marked in Ss deprived for longer periods. A period of 4 or 8 days of deprivation was sufficient to cause cortical changes comparable to those seen after much longer periods of deprivation. Neuronal receptive fields activated through the deprived eye appeared to lose much or all of their stimulus selectivity before disappearing altogether. Many neurons in the kittens deprived for 1, 2, or 4 days possessed normal orientation-selective receptive fields in the eye that had been open and nonoriented or poorly selective receptive fields in the eye that had been closed. The changes in cortical binocularity were accompanied by changes in the distribution of physiologically revealed ocular-dominance columns. A close correlation between the changes in cortical binocularity and the changes in geniculate cell size supported the hypothesis that these effects share a common origin, perhaps related to the competition for terminal space in the cortex which was reflected in the physiological and morphological changes in the relative extent of the cortical regions dominated by the 2 eyes in deprived animals.