Two differential pairing procedures were applied in the primary visual cortex of anesthetized and paralyzed kittens and cats, to produce changes in ocular dominance and interocular orientation disparity (IOD) during the time of recording of a single neuron. A first experiment was devised to demonstrate plasticity in the balance of monocular responses. The visual activity of the cell was driven iontophoretically to either a "high" or a "low" level, depending on the ocularity of the visual stimulation. Ocular dominance measurements before and after pairing revealed significant long-lasting changes in 33% of cases. Relative ocular preference shifted in most cases (87.5%) in favor of the reinforced eye. Similar proportions of significant changes were observed in kitten and adult cortex. The amplitude of the functional modifications was not significantly related with age, although the largest changes in ocular dominance were recorded at the peak of the critical period. The second experiment more specifically addressed the plasticity of binocular interaction. The activity of a binocular cell was driven iontophoretically to either a "high" or a "low" level, depending on the orientation disparity between two oriented stimuli, presented simultaneously and separately in the receptive field of each eye. Significant long-lasting changes in binocular responses were induced in 40% of cases. The relative IOD preference generally shifted (67%) in favor of the reinforced disparity. In half of the modified cells, functional changes were expressed only in the dichoptic viewing condition used during the pairing procedure. These functional modifications of binocular integration, demonstrated at the cellular level, are analogous to those induced by global manipulations of the visual environment (Hubel and Wiesel, 1970; Shinkman and Bruce, 1977). They are interpreted as evidence for synaptic plasticity. Our results support the hypothesis that covariance levels between pre- and postsynaptic activities determine the sign and the amplitude of changes in synaptic efficacy.