In 6 week old dark reared kittens, a visual monocular exposure for 6 hrs. elicits major functional modifications in the primary visual cortex. Most cells become orientation selective and are activated through the open eye only. However, if a unilateral section of the ophthalmic branch of the fifth nerve is performed 1 week before the monocular exposure, these modifications are less pronounced, particularly when carried out on the same side as the open eye. Moreover, if the ophthalmic branch has been cut bilaterally, visual monocular exposure has almost no effect: cortical cells remain non selective to orientation and binocularly activated.