The development of the lateral geniculate nucleus has been studied systematically in Nissl preparations from a series of cats that ranged in age from newborn to adult. In addition, preliminary observations are reported at two stages of fetal development. It was found that laminae develop in the lateral geniculate nucleus near the time of birth and continue to differentiate during the first postnatal week. During development the major axis of the lateral geniculate rotates approximately 180 degrees in the sagittal plane. The rotation begins prenatally and is not completed until after the twentieth postnatal week. The volume of the lateral geniculate was computed at different ages and it was determined that during the first postnatal month the nucleus attains two-thirds of its adult size. However, the rate of growth declines markedly thereafter, and final volume, like final position, is not achieved until late in development. The cross-sectional areas of lateral geniculate neurons were measured at four locations in the nucleus in each animal. The locations represented the following parts of the visual field: the paracentral and inferior peripheral fields in the binocular segment of lamina A; the monocular segment of lamina A; and the paracentral field in lamina A1. Neurons in each of these locations grow at approximately the same rate and are essentially fully grown by 56 days. Cell size histograms show that more large cells are found in lamina A1 and more small cells in the monocular segment than elsewhere in the dorsal laminae. Unlike the retina, there appears not to be a gradient of development in the lateral geniculate nucleus from center to periphery, at least in terms of cell body size at the ages studied. On the contrary, that part of the lateral geniculate nucleus which represents the paracentral visual field is the last segment in the dorsal laminae to achieve a mature cell size distribution. Finally, a discrete class of small spindle-shaped neurons was observed in the lateral geniculate nucleus ventral and caudal to the C laminae during the first two postnatal weeks. These cells possess a leading and trailing cytoplasmic process and are distinctly different from cells in the main laminae. It is suggested that these spindle-shaped cells may be neurons that are still in the process of migration or differentiation in the postnatal animal.