The distribution of GABA-immunoreactive neurons was visualized in the basal ganglia of the squirrel monkey (Saimiri sciureus), by using a highly specific antiserum raised against GABA-glutaraldehyde-lysyl-protein conjugate and revealed by the indirect peroxidase-antiperoxidase immunohistochemical method. In the dorsal striatum, GABA-immunoreactive nerve cell bodies were small to medium in size (sectional area ranging from 90 to 125 microns2), but some larger ones (500-600 microns2) were also found. These cells displayed no obvious clustering but were significantly more numerous in the caudate nucleus than in the putamen; their number was also markedly greater at caudal than at rostral striatal levels. A moderate number of evenly distributed positive axon terminals were visible in both the caudate nucleus and the putamen. In the ventral striatum, GABA-immunoreactive nerve cell bodies and axon terminals were seen in fair number within the nucleus accumbens and in the deep layers of the olfactory tubercle. Many positive terminals but no somata were found in the islands of Calleja. In the globus pallidus, virtually all nerve cell bodies were GABA-immunoreactive and the neuropil exhibited a multitude of positive terminals. In the substantia innominata, clusters of small, globular GABA-immunoreactive somata were scattered among aggregates of larger, nonimmunoreactive neurons belonging to the nucleus basalis, and the whole region showed a low to moderate number of evenly spread GABA-positive terminals. In the subthalamic nucleus, nerve cell bodies were generally surrounded by several GABA-positive terminals but were not themselves immunoreactive. The substantia nigra showed many GABA-immunoreactive somata, which predominated in the pars lateralis and diminished progressively in number along the lateromedial axis of the pars reticulata. These cells formed a rather pleomorphic group comprising round, fusiform, or polygonal elements of relatively large size (sectional area ranging from 200 to 800 microns2). In the pars compacta and ventral tegmental area, a few GABA-immunoreactive neurons of small size were dispersed among larger, unreactive neurons. In both pars lateralis and pars reticulata of the substantia nigra, the number of GABA-positive terminals was high and their distribution was rather uniform; a smaller number were visible in the pars compacta of the substantia nigra and in the ventral tegmental area. The present results demonstrate that GABA-containing neurons are widely and heterogeneously distributed in the various components of the squirrel monkey's basal ganglia.(ABSTRACT TRUNCATED AT 400 WORDS)