Experiments have been carried out to ascertain whether intracortical inhibitory processes influence the ocular dominance of monocularly dominated cells in the primary visual cortex of the normal cat. The GABA antagonist bicuculline has been iontophoretically applied to the cells studied to produce a localised block of inhibitory mechanisms acting on them. The ocular dominance of these cells was tested before, during, and after bicuculline application. In a sample of 42 cells studied, approximately 50% (19) showed a significant change in ocular dominance during bicuculline application. Some exclusively monocular cells became equally driven by either eye during bicuculline application. All the effects were reversible. Receptive field properties revealed in the non-dominant eye were not identical to those in the dominant eye. Evidence is presented to suggest that for some of the cells there is a selective GABA-mediated inhibitory process suppressing the non-dominant eye input. The possible implications of these data are discussed.