The anterograde transport of 3H proline and of horseradish peroxidase has been used to study the retinogeniculate pathway in normal adult ferrets and in young ferrets during postnatal development. the lateral geniculate nucleus in adults shows a characteristic "carnivore" pattern, with layers A, A1, C, C1, C2, and C3, and a medial interlaminar nucleus recognizable either cytoarchitectonically or on the basis ofth retinogeniculate innervation. In addition, there is a well-defined, rather large perigeniculate nucleus. At birth the lateral geniculate nucleus is unlaminated and essentially all parts are reached by afferents from both eyes. The crossed component is by far the larger. It extends from the optic tract medially well into the perigeniculate field, in contrast to the uncrossed component which barely reaches the perigeniculate field. During the first 3 postnatal days the uncrossed fibers restrict their arbors to a small posterior and medial region, the precursor of the biocular segment of the nucleus. The crossed fibers gradually retreat from the region within which the uncrossed fibers have concentrated. Between the fourth and eighth postnatal days the field occupied by the ipsilateral component expands again to form a major focus that will define lamina A1 and a minor focus that will define C1. At this stage the crossed and the uncrossed fibers overlap at the borders of lamina A1 and the whole region of lamina C1 is also occupied by arbors of the crossed component. The perigeniculate field becomes clearly distinguishable from the lateral geniculate nucleus and the medial interlaminar nucleus is becoming clearly recognizable between days 3 and 8. Between days 8 and 15 the cytoarchitectonic borders between layers A and A1 become clearly defined, but the retinogeniculate axons from each eye still extend across this border. These axons retreat into their appropriate lamina after the 15th postnatal day an the nucleus reaches its essentially adult structure by about the fourth postnatal week. Segregation of retinofugal axons in the C layers occurs after segregation in the A layers, but many of the cells within the C layers show signs of cytological maturity earlier than those of the A layers. The nucleus undergoes a series of migrations and changes of shape as the ipsilateral and contralateral components become segregated. Whereas in teh newborn the nucleus is roughly comma-shaped and on the lateral aspect of the dorsal thalamus, in the adult it is "L"-shaped and mainly on the posterior aspect of the dorsal thalamus.