The technique of injecting single retinogeniculate fibers with horseradish peroxidase enables the terminal arbors of physiologically identified axons to be fully characterized morphologically. We have used this technique to study the postnatal development of retinal X and Y cell arbors within the cat's lateral geniculate nucleus, and the plasticity of these arbors following a variety of manipulations that perturb normal development. These experiments suggest quite specific sequences and mechanisms for the development of individual X and Y retinogeniculate axons. Retinal X axons appear to innervate the lateral geniculate nucleus before Y axons do, and are probably specified innately to their appropriate target lamina A or A1. By 3–4 weeks postnatally, X axons from each eye develop exuberant terminal arbors within the A laminae that by 12 weeks get pruned to the narrow adult form by later developing Y axon arbors from the same eye. The Y arbors progressively expand to form their characteristic broad terminal zones during this period. The laminar location of Y arbors depends on interactions between axons from the two eyes, and their transverse extent on the presence of normal afferent activity in retinogeniculate fibers.