The molecular composition of the γ-aminobutyric acid type A (GABA_A) receptors, strongly determines both the electrophysiological and pharmacological properties of these receptors. Pharmacologically three types of GABA_A receptors have been identified according to their affinities for the imidazopiridazine zolpidem; type I, with high affinity, type II_M, with medium affinity and type II_L with low affinity for this compound. However, a total of fifteen subunits belonging to GABA_A receptors (α_1-6, β_1-3, γ_1-3, δ and ε) have been cloned. A minimum of αβγ subunit combination is needed to resembled all the properties of the native GABA_A receptors, suggesting that a high theoretical number of GABA_A receptors could exist. Thus, the predicted number of GABA_A receptors from molecular data is higher than the pharmacological classification. The absence of specific tools, as isotype selective compounds, has strongly limited the advance in the discovery of new pharmacologically different GABA_A receptors. Furthermore, in the last years molecular data have revealed the existence of native GABA_A receptors made up by two different alpha subunits in the same receptor complex, introducing a new level of complexity in the molecular composition of the GABA_A receptors. In this case, both the pharmacological and physiological properties need to be determined.