Immunohistochemical studies undertaken with a highly specific antiserum raised against gamma-aminobutyric acid (GABA)-glutaraldehyde-lysyl-protein conjugate showed that cell bodies of the subthalamic nucleus in the squirrel monkey (Saimiri sciureus) were closely surrounded by several GABA-positive terminals but were not themselves immunoreactive. In contrast, after incubation with a monoclonal antibody directed against carbodiimide-fixed glutamate, virtually all cell bodies of the subthalamic nucleus displayed an intense immunoreactivity. They were surrounded by various neuronal processes that also stained for glutamate. These results suggest that the neurons of the subthalamic nucleus in primates utilize the excitatory neurotransmitter glutamate instead of the inhibitory neurotransmitter GABA.