In mammals, monocular deprivation performed during the early stages of postnatal development (critical period) dramatically affects the functional organization of the visual cortex. Since the early work of Hubel and Wiesel, the effects of monocular deprivation are accounted for by the fibers driven by the two eyes competing for the control of cortical territories. In cat and monkey striking structural changes accompany the functional effects of monocular deprivation. Also, in the rat, monocular deprivation causes functional alteration at the level of visual cortex; no structural correlates of these effects, however, have so far been described. Parvalbumin is a calcium binding protein that in the neocortex colocalizes with a subpopulation of GABAergic neurons. Here we report that in the rat monocular deprivation results in a dramatic reduction of parvalbumin-like immunoreactivity in the visual cortex contralateral to the deprived eye. This effect is due to competitive phenomena and not to visual deprivation itself, it is restricted to the binocular portion of the visual cortex and neither binocular deprivation, nor dark rearing can induce it. We conclude that parvalbumin-like immunoreactivity is a useful immunohistochemical marker for the effects of monocular deprivation in the rat visual cortex.