A differential pairing procedure was applied in vivo to individual neurons in the primary visual cortex of anesthetized paralyzed cats, in order to produce changes in their relative orientation preference. While we recorded from a single cell, its visual response to a light bar was driven iontophoretically to a "high" level when stimulating with an initially nonpreferred orientation (S+), and alternately reduced to a "low" level when stimulating with the preferred orientation (S). This associative procedure was devised to test the possible role of neuronal coactivity in controlling the plasticity of orientation selectivity. Among 87 cells tested, 35 (40%) showed significant long-lasting changes, either in the relative orientation preference for the two "paired" stimuli S+ and S-, in the global orientation tuning profile, or in both. Measurements of relative orientation preference demonstrated significant effects in 27 cells (31%), all in favor of the positively reinforced orientation (S+). Modifications of orientation selectivity (studied over the entire orientation spectrum in 45 of the conditioned cells) usually consisted (21 out of 25 modified cells) of a competitive reorganization of the orientation tuning curve: the preferred orientation shifted toward S+, and a loss of relative visual responsiveness was observed for orientations close to the negatively reinforced orientation (S-). The largest changes were found in deprived kittens at the peak of the critical period, although the probability of inducing a significant change studied during the first year of postnatal life was independent of age. These functional modifications demonstrated at the cellular level are analogous to those induced by a global manipulation of the visual environment, when only a restricted spectrum of orientations is experienced during the critical period. Our results support the hypothesis that covariance levels between pre- and postsynaptic activity determine the sign and the amplitude of the modification of efficacy of cortical synapses.