AbstractThe subthalamic nucleus (STN) is considered a homogeneous structure composed essentially of projection neurons that exert a profound glutamate‐mediated excitatory influence upon the main output structures of the basal ganglia. It is currently the most efficient target for deep brain stimulations designed to alleviate symptoms of Parkinson's disease. STN neurons were analyzed by applying stereological methods and single/double‐immunostaining procedures to postmortem material obtained from normal individuals. Besides a multitude of closely packed projection neurons (∼24.7 μm in diameter), the human STN (mean volume, 174.5 ± 20.4 mm3; total neuronal density, 239.5 ± 31.9 × 103) contained smaller neurons (∼12.2 μm), which displayed glutamic acid decarboxylase (GAD)65/67 immunoreactivity and shared the morphological features of interneurons described in Golgi studies of primate STN. These putative γ‐aminobutyric acid (GABA)ergic interneurons accounted for 7.5% of the total neuronal population of the STN. Although present throughout the nucleus, they were significantly more numerous in its posterior‐ventral‐medial sector, which belongs to the limbic/associative functional territory. Many projection neurons located dorsolaterally in the STN showed parvalbumin immunoreactivity and others lying ventromedially displayed calretinin immunostaining, but none of the GAD‐positive interneurons expressed these calcium‐binding proteins. Although less abundant than projection neurons, GABAergic interneurons might play a important role in the intrinsic organization of the STN. The morphological and chemical heterogeneity of the human STN reported here might have important implications in the functional organization of the basal ganglia. © 2005 Movement Disorder Society