Large inactive regions of the retina of adult cats were produced by the novel method of inducing monocular retinal detachment. Within a few hours, neurones throughout the detachment projection zone in primary visual cortex (55-136 mm2), including some > 4.5 mm from its boundary, were found to have large receptive fields displaced onto intact retina. The new receptive fields of some neurones represented shifts of up to 9 mm across the retinotopic representation. For these rapid changes to occur pre-existing viable circuits must provide a cortical locus with inputs from a wide extent of the retina. Receptive fields, and the retinotopic map, for stimulation of the other eye were unchanged.