Site-selective drug effects on the ion-channel activities of γ-aminobutyric acid type A (GABAA) receptors are evaluated by using a nanovesicle-carbon nanotube hybrid device. Here, nanovesicles containing GABAA receptors are immobilized on the channel region of a carbon nanotube field-effect transistor. The receptor responses of this hybrid device to GABA are detected with a high sensitivity down to ∼1 aM even in the presence of other neurotransmitters. Further, sensitivity differences between two GABAA-receptor-subunit compositions of α5β2γ2 and α1β2γ2 are assessed by normalizing the dose-dependent responses obtained from these hybrid devices. Specifically, the GABA concentration that produces 50% of maximal response (EC50) is obtained as ∼10 pM for α5β2γ2 subunits and ∼1 nM for α1β2γ2 subunits of GABAA receptor. Significantly, the potency profiles of both antagonist and agonist of GABAA receptor can be evaluated by analyzing EC50 values in the presence and absence of those drugs. A competitive antagonist increases the EC50 value of GABA by binding to the same site as GABA, while an allosteric agonist reduces it by binding to a different site. These results indicate that this hybrid device can be a powerful tool for the evaluation of candidate drug substances modulating GABA-mediated neurotransmission.