If a young monkey or kitten is monocularly deprived for a period of days of weeks, the ocular dominance stripes or patches formed in layer IV of the visual cortex by the geniculo-cortical afferents driven by that eye become smaller, while the patches formed by afferents from the other, experienced eye, spread out and increase in size. One explanation for this effect is that it results from a disturbance of competitive process which, during the first weeks of life, guides a "sorting out" of the initially intermixed right and left eye inputs into complementary, largely non-overlapping territories. One feature of this process may be a local interaction between right and left eye synapses in which like synapses reinforce each other's growth rates and cause rejection of the other eye's synapses. If this is the case, then the effect of monocular deprivation on the relative sizes of the two sets of columns can be explained by supposing that the strengths of the effects exerted by the deprived eye are reduced. This explanation has a testable consequence: if both eyes are deprived of vision then each eye should be made less effective in eliminating the other eye's inputs, and the overall rate at which the ocular dominance columns form should be decreased. Although LeVay et al. found that columns were present in a 7-week old monkey reared in the dark from the age of 3 days, this result does not necessarily imply that the rate of column formation had been normal, because in normal monkeys the columns are well developed by 3 or more weeks of age. I report here the results of transneuronal autoradiography in cats, which show that columns, as revealed anatomically, are undetectable in most parts of the visual cortex of cats reared in the dark for periods of up to 20 weeks, implying that visual experience is necessary for their proper formation.