Recent investigations have suggested that the morphological effects of monocular deprivation can be explained by a developmental competitive interaction between the pathways from the two eyes. This study presents evidence in the tree shrew for binocular competition and for an unequal effect of such competition on the different layers of the lateral geniculate nucleus. The effects of monocular deprivation were evaluated by comparing cell size changes in the binocular and monocular segments of the lateral geniculate nucleus in three tree shrews raised with one eye sutured. In two of these animals the open eye was injected with 3H proline in order to identify accurately geniculate layers innervated by the non-deprived eye. Cell sizes in three normal animals and one monocularly enucleated animal were measured for comparison. The results show the following main effects: First, that monocular deprivation significantly changes cell size in the binocular but not the monocular segment of the geniculate nucleus. Comparisons with cell size in normal animals indicates that non-deprived cells may grow in response to deprivation. Second, that cell size in geniculate lamina 3 is not affected by monocular deprivation, suggesting that cells in this layer are morphologically or functionally secluded from competitive interactions affecting the other layers. Finally, that monocular enucleation in the adult tree shrew affects all parts of the geniculate nucleus including layer 3 and the monocular segment, demonstrating that these parts of the geniculate nucleus are responsive to lack of retinal innervation.