Previous studies have demonstrated that removal of inputs from the nondeprived eye of monocularly deprived cats produces a recovery from some of the abnormalities caused by the deprivation. The present experiments examined whether the loss of Y-cells that occurs in the deprived laminae of the lateral geniculate nucleus (LGN) also can be reversed by this manipulation and, if so, what are the necessary and sufficient conditions for reversal to occur. Single cell recordings were made in LGN laminae A and A1 of 31 cats in a variety of rearing conditions, and the cells were classified as X, Y, or mixed on the basis of responses to a battery of tests. In normal cats, a mean of 39.1% of the LGN cells were Y-cells. In agreement with previous studies, monocular deprivation produced a reduction in the proportion of Y-cells encountered (18.1%). Removing the nondeprived eye at 4 months of age did not produce an increase in the proportion of Y-cells if the deprived eye remained closed for 3 months (16.9%). However removing the nondeprived eye at 4 months and opening the deprived eye for 3 months did produce an increase in the proportion of Y-cells (40.2%). Thus, the loss of Y-cells in the LGN that results from rearing with monocular deprivation can be reversed at 4 months of age; however, it is necessary to allow the deprived eye visual experience. In a second experiment, we assessed the age limits and the conditions that are both necessary and sufficient for the recovery of Y-cells. Removing the nondeprived eye at 12 to 16 months of age and allowing the deprived eye 3 months of visual experience still produced a significant increase in the proportion of Y-cells (30.3%). Thus, some reversal can occur well into adulthood. However, opening the deprived eye and simply closing the nondeprived eye (reverse suture) for 3 months in adult monocularly deprived cats did not produce a reversal of the LGN Y-cell loss (14.3%). Thus, it is necessary both to allow the deprived eye visual experience and to remove the inputs from the nondeprived eye to produce a recovery of LGN Y-cells. Simply closing the nondeprived eye is not sufficient, even if the deprived eye is opened. Taken together with previous studies, these results also indicate that there is a dissociation between functional recovery in LGN and striate cortex and between recovery of LGN cell size and the functional class of Y-cells following removal of the nondeprived eye.