The distribution of the high affinity gamma-aminobutyric acid (GABA) receptor labeled by [3H]muscimol, has been studied in the rat brain by light microscopic autoradiography. Receptors in slide-mounted tissue sections were labeled in vitro with [3H]muscimol. Most of the gray matter areas presented grain densities significantly higher than background or white matter areas. Wide variations in receptor densities were found between different brain areas and nuclei. Areas with very high grain densities are the granule cell layer of the cerebellum, external plexiform layer of the olfactory bulb and nuclei of the thalamus, such as the ventral nucleus, lateral nucleus and dorsal geniculate body. The molecular layer of the hippocampus and the external (I-IV) layers of the cortex are also rich in GABA receptors. The basal ganglia have moderate concentrations of receptors, while the pons, medulla and brainstem have only low concentrations of autoradiographic grains. These distributions are discussed in correlation with the known distribution of GABAergic terminals and the presence of inhibitory GABAergic mechanisms.