In an effort to determine subunit compositions of in vivo GABAA receptors, the cellular localization of 13 subunit encoding mRNAs (alpha 1-alpha 6, beta 1-beta 3, gamma 2-gamma 3, delta) was determined in the rat olfactory bulb and cerebellum. Cerebellar granule cells expressed significant quantities of alpha 1, alpha 6, beta 2, beta 3, gamma 2, and delta mRNAs. They contained very much lower levels of alpha 4, beta 1, and gamma 3 mRNAs, and the alpha 2, alpha 3, alpha 5, and gamma 1 genes appeared to be silent. Purkinje cells contained only the alpha 1, beta 2, beta 3, and gamma 2 mRNAs. Putative Bergmann glial cells were found to contain the gamma 1 mRNA and possibly the alpha 2 mRNA. In the molecular layer, only the alpha 1, beta 2 and gamma 2 mRNAs were expressed in stellate/basket cells. The alpha 3 probe hybridized weakly to targets in the molecular layer. The inferior olivary nucleus contained significant quantities of alpha 2, alpha 4, and gamma 1 transcripts, with the alpha 1, alpha 3, beta 2, beta 3, and gamma 2 mRNAs also present. In the olfactory bulb, mitral cells were found to express the alpha 1, beta 1, beta 2, beta 3, and gamma 2 mRNAs strongly and the alpha 3 mRNA weakly. Tufted cells contained alpha 1, alpha 3, beta 2, beta 3, and gamma 2 mRNAs and, occasionally, the alpha 2 mRNA. In the internal granule cells the alpha 2, alpha 4, alpha 5, beta 3, and delta mRNAs were all present. Low levels of alpha 3, gamma 1, gamma 2, and gamma 3 mRNAs were also noted in these cells. Periglomerular cells expressed low levels of alpha 2, alpha 3, alpha 4, beta 2, beta 3, gamma 1, gamma 2, and gamma 3 mRNAs. No alpha 6 mRNA was present in the olfactory bulb. Correlations that are general ones from other brain regions are the colocalizations of alpha 1 beta 2, alpha 2 beta 3, and alpha 4 delta mRNAs. In both the olfactory bulb and cerebellum, alpha 1 beta 2 gamma 2 receptor cores are probably employed. The delta-subunit mRNA appears to codistribute with alpha- subunit mRNAs (alpha 4 and alpha 6) associated with GABAA subunits that fail to bind benzodiazepine agonists.