GAD (E.C. 4.1.1.15), the endogenous biosynthetic marker enzyme for GABA, was localized in rat globus pallidus and nucleus entopeduncularis (pallidum) by peroxidase-antiperoxidase immunocytochemistry. In the two nuclei we observed numerous GAD-like immunoreactive boutons containing large pleomorphic vesicles. These boutons formed axodendritic and axosomatic synaptic contacts mostly of the symmetric type. Following colchicine injection near the border of globus pallidus or nucleus entopeduncularis, the vast majority of both larger and smaller neurons exhibited GAD immunoreactivity and were contacted by numerous GAD-immunoreactive boutons. These data demonstrate that pallidal neurons are of predominately GABA-ergic nature and receive an extensive GABA-ergic input. Thus, the globus pallidus and the nucleus entopeduncularis major output stations of the basal ganglia may represent sites of extensive GABA-mediated inhibition of GABA-ergic inhibitory neurons.