Previous studies have shown that suturing one eyelid closed in a newborn kitten results in profound changes in the development of the visual system. Among these is a retardation in the growth of neurons in the layers of the dorsal lateral geniculate nucleus receiving retinal input from the closed eye. Moreover, the greatest effect appears to be in the largest neurons. The present study examines the effects of monocular deprivation on the perikaryal size of a select group of small lateral geniculate neurons. GABAergic neurons, that may be interneurons. These cells were selectively labeled by an antiserum to glutamic acid decarboxylase (GAD) and immunocytochemical methods. The results demonstrate that GAD+ neurons are among the smallest in the lateral geniculate nucleus and that they are insensitive to the effects of monocular deprivation. That is, GAD+ neurons in deprived laminae A and A1 are similar in size to those in the corresponding, nondeprived laminae. These findings are consistent with the hypothesis that GAD+ neurons are interneurons and therefore not subject to binocular competition in the visual cortex. This interpretation, however, is complicated by additional studies of the postnatal development of GAD+ neurons which reveal that GAD+ neurons grow to their adult size relatively early, before the onset of the critical period. Thus the insensitivity of the perikarya of GAD+ neurons to monocular deprivation may be attributable to their precocious growth.