The distribution of the short (gamma 2S) and long (gamma 2L) subunits of the GABAA receptors in the rat brain has been revealed by light microscopy immunocytochemistry with novel subunit-specific antibodies (anti-gamma 2S and anti-gamma 2L). We have also used other subunit- specific antibodies including anti-gamma 2IL2 (which recognizes both gamma 2S and gamma 2L), anti-alpha 1COOH, and the monoclonal antibody 62–3G1 to beta 2/3 for comparing the regional and cellular distribution of the most abundant GABAA receptor subunits in the rat brain. The distributions of gamma 2S and gamma 2L immunoreactivities are similar throughout the brain although the relative intensity of both signals varies depending on the brain area and neuronal type. In the hippocampus, cerebral cortex, and olfactory bulb (particularly mitral, periglomerular, and tufted neurons), gamma 2S was more abundant than gamma 2L. In contrast, the inferior colliculus, medulla, and the cerebellar Purkinje cells displayed more gamma 2L than gamma 2S immunolabeling. An important difference in the distribution of the various subunits was found in cerebellum: gamma 2S and gamma 2L were predominantly localized in the molecular layer, whereas alpha 1 and beta 2/3 were more abundant in the granular layer. In the thalamus, gamma 2L and gamma 2S were less abundant than either alpha 1 or beta 2/3 subunits. The results showed that there is colocalization of gamma 2S and gamma 2L subunits in some brain areas and neuronal types, as well as areas of mismatch. Colocalization and mismatches were also found among alpha 1, beta 2/3, and gamma 2, probably resulting from the heterogeneity in the subunit composition of the GABAA receptors through the brain.