We have investigated the GABAA receptor mRNA composition in 13 cell lines, using 13 subunit-specific oligo-primers (alpha 1–6, beta 1–3, gamma 1–3, and delta) and reverse transcriptase PCR amplification. Cell lines (B35, B65, B103, B104, RINm5F, Rat1, PC12, C6, C17, C27, beta TC3, NB41A3, AtT-20), derived from diverse tissue origins, were investigated in order to identify homogeneous cellular sources with distinctive GABAA receptor subunits. Fifteen GABAA receptor subunits have been cloned from mammalian tissue (those listed above plus the retinal subunits rho 1 and rho 2). This multiplicity of GABAA receptor subunits underlies the diverse pharmacology of the GABAA receptor. Attempts to understand the regulation and pharmacology of individual subunits and of the heterooligomeric receptor combinations have been impeded by a lack of pure populations of cells expressing GABAA receptor subunits. Permanent cell lines provide such a resource. Each GABAA receptor subunit mRNA, alpha 1–5, beta 1–3, gamma 1–3, and delta, was detected in at least one cell line. All cell lines examined contained detectable levels of at least one GABAA receptor subunit mRNA. Each cell line contained distinctive combinations of subunit mRNAs. None of the cell lines examined contained detectable amounts of alpha 6 mRNA. These cell lines, which transcribe GABAA receptor subunit mRNAs, provide useful cellular sources for transcriptional and pharmacological studies. Our data also suggest that endogenous GABAA receptor subunit mRNAs may be present in cells that are routinely used for transfection studies, and that this expression might confound interpretation of the studies. In the following companion article, we have looked for functional GABAA receptor Cl- ion channels in these cell lines, using the patch-clamp technique (Hales and Tyndale, 1994).