1. We examined the terminal arbors of single, physiologically identified retinogeniculate X and Y axons in 13 adult cats raised from birth with binocular lid suture. We recorded in the optic tract from 146 retinogeniculate axons. We studied the response properties of each axon encountered and attempted to penetrate it for labeling with horseradish peroxidase. 2. We attempted to classify each retinogeniculate axon as X or Y on a standard battery of tests. We thus identified 46 X and 91 Y axons; 5 axons had unusual response properties, and 4 axons were lost before they could be adequately identified. The X and Y axons had response properties that were completely normal by our criteria. The 5 unusual axons exhibited linear spatial and temporal summation, which is a property of X cells, despite all of their other tested response properties being consistent with those of Y cells. 3. We achieved complete, dark labeling of 13 X and 13 Y axons that form the data base for all of our qualitative and quantitative morphological observations. All of these labeled axons had response properties entirely normal for their X or Y class. Nine of the labeled X axons arise from the contralateral retina and 4 from the ipsilateral retina, whereas the respective numbers for the Y axons are 8 and 5. 4. Each of the individual retinogeniculate X axons form terminal arbors that appeared essentially normal in terms of location within geniculate lamina A or A1, shape, volume, and number of terminal boutons. 5. In contrast, the retinogeniculate Y axons form clearly abnormal arbors with diminished projections, both in terms of bouton numbers and arbor volumes. For Y axons from the contralateral retina, a roughly normal arbor is formed in the C-laminae, despite greatly diminished or absent projections formed in lamina A, something never seen in normal cats. For Y axons from the ipsilateral retina, the projections to lamina A1 are also diminished, and the arbors there are all limited to the ventral half of the lamina, a pattern rarely seen for normal Y axons. 6. The selective reduction in retinogeniculate Y axon arbors in these binocularly lid-sutured cats is consistent with similar observations reported for monocularly lid-sutured and strabismic cats but is quite different from the apparently normal development of retinogeniculate axon arbors in cats raised in complete darkness.(ABSTRACT TRUNCATED AT 400 WORDS)