The cat retinogeniculocortical pathways are organized chiefly into two parallel independent neuronal streams, one involving X-cells of the retina and lateral geniculate nucleus, and the other, Y-cells. Development of the Y-cell pathway is more seriously affected by visual deprivation than is the X-cell pathway and we reasoned that some insight into the underlying mechanisms of these effects could be gained from studies of normal development. We therefore injected horseradish peroxidase into physiologically identified X- and Y-cell retinogeniculate axons to examine the postnatal development of their terminations in kittens. As we report here, at 3-4 weeks of age, most optic tract axons can be identified physiologically as members of the X- or Y-cell class. X-cell terminal fields in lamina A or A1 are wider at 3-4 weeks than they are in adults, while Y-cell terminal fields are narrower than in adults. During the second and third postnatal months, X-cell terminal arbors progressively contract while Y-cell arbors expand so that, by 12 weeks of age, the adult pattern is seen. These data, and the results of our earlier study of the effects of monocular lid suture on these terminal arbors, suggest that enlargement of Y-cell terminations in geniculate lamina A or A1 during development may be accompanied by competitive pruning of X-cell terminations within these same laminae.