Neuronal cell bodies in the lateral geniculate nucleus of normal and of monocularly-deprived Siamese cats have been measured. Seventeen normally reared Siamese cats, ranging in age between 20 and 120 days, were used to determine rates of normal geniculate cell growth. A second group of five adult Siamese cats reared from bith with the lids of one eye closed were used to study the effects of monocular visual deprivation upon geniculate cell size. For each of the normal and visually deprived Siamese cats, the cross-sectional areas of 600 lateral geniculate cells were measured from camera lucida drawings of Nissl preparations. During normal development the geniculate cells rapidly increase in size during the first postnatal month of life and reach their adult size sometime between days 28 and 56. While this course of geniculate cell growth is similar to that seen in normally pigmented cats, the pattern of change seen after monocular deprivation is quite different in Siamese cats from that found in normally pigmented cats. In Siamese cats the regions of the nucleus receiving a contralateral projection from the deprived eye appear to be shielded from the effects of binocular competition. Cells throughout lamina A and in the abnormal, contralaterally innervated segment of lamina A1 show only about a 10% reduction in cell size. There are no noticeable differences between the parts of lamina A in the binocular and monocular segments of the nucleus. Cells in the ipsilaterally innervated segment of lamina A1, in contrast, show deprivation-induced changes that average 27.1%. Two mechanisms are proposed to explain why some geniculate cells in Siamese cats appear to be shielded from binocular competition: one depends on possible interactions between geniculo-cortical cells lying in adjacent parts of the same geniculate lamina, and the other depends on an anatomical segregation of the cell type ("Y-cells") most heavily affected by the binocular competition. Each proposed mechanism is related to earlier observations on monocularly deprived, normally pigmented cats.