In many mammalian species, rearing with one eyelid closed produces a loss of vision in the deprived eye and a change in cell size in the lateral geniculate nucleus (LGN). In cats, the reduction in the size of deprived LGN cells has been correlated with a loss of one functional class of cells, Y cells. In primates, such as galago, LGN cells also exhibit marked changes in size with deprivation. In the present study we recorded from single cells in the LGN of monocularly deprived galagos to determine if such changes in cell size would be accompanied by changes in physiological properties. The results revealed no alterations in the distribution or functional properties of any cell class. The differences in the effects of monocular deprivation on the function of LGN cells in cats and primates are most easily explained by a fundamental difference in visual system anatomy. In cats, different classes of retinal afferents (X vs. Y) are in a position to compete for postsynaptic LGN neurons: in primates, segregation of cell classes into different layers may preclude such developmental interactions.