We have studied the relative importance of visual and nonvisual signals to striate neurons in causing the loss of cortical binocularity that occurs when young kittens are subjected to brief episodes of stabismus. In kittens kept in a normally illuminated colony, the reduction in binocularity was the same whether the strabismic was induced optically or surgically. When surgically strabismic kittens were kept in total darkness, cortical binocularity was unchanged. We conclude that anomalous visual input is both necessary and sufficient to cause loss of binocularity in strabismic kittens and that nonvisual (proprioceptive) input from the orbit has no part in producing this effect.