Single unit, extracellular recordings were made in the medial interlaminar nucleus (MIN) of adult cats and of kittens at 2, 4, 6, 8, 12, and 16 weeks of age. MIN is part of the dorsal lateral geniculate nucleus, and nearly all of its recorded neurons are Y-cells in adult cats. We find that Y-cells in the MIN of younger kittens have long latencies to optic chiasm stimulation, large receptive fields without surrounds, low spatial and temporal resolutions, and nearly absent spontaneous activity. Responses to hand-held light stimuli are typically poor and inconsistent. Also, most of these immature Y-cells exhibit only linear response properties when tested with a counterphased modulated grating pattern. All of the above characteristics gradually develop to adult values over several months of the postnatal period. The percentage of cells with nonlinear response properties becomes adult-like near 8 weeks of age, while both spatial and temporal resolutions are still developing at 16 weeks of age. Latencies and receptive field center sizes achieve adult values at about 12 weeks of age. The immature properties of these neurons are similar to those recorded from abnormal cells in the area of the medial interlaminar nucleus innervated by the deprived eye of monocularly deprived adult cats. This indicates that the adult deprived Y-cells probably fail to develop and thus retain their immature properties.