GABAARs, the major inhibitory neurotransmitter receptors in the brain, belong to the superfamily of pentameric ligand-gated ion channels, each containing five identical/homologous subunits arranged pseudosymmetrically around a central ion channel. For α1β3γ2 GABAARs, the most common subtype, subunits are ordered βαβαγ, with transmitter binding sites at the β+-α- interfaces in the extracellular domain. Two homologous but pharmacologically distinct classes of general anesthetic binding sites have been identified in the GABAAR transmembrane domain by use of photoreactive general anesthetics. In a α1β3γ2 GABAAR, azietomidate binds with high affinity (IC50 = 3 μM) and 100-fold selectivity to β+-α- sites, while a mephobarbital analog, R-mTFD-MPAB, binds to α+-β-/γ+-β- sites (IC50 = 1.3 μM) with 60-fold selectivity. Competition photolabeling assays using these photoprobes have identified the selectivity and affinities of other general anesthetics for these sites. Now, extending this approach, we characterized competitive and allosteric interactions of non-anesthetic GABAAR modulators with these sites. Ivermectin, a GABAAR potentiator known to bind subunit interface sites in homologous receptors, is ∼100-fold selective for R-mTFD-MPAB sites (IC50 = 20 nM) compared to azietomidate sites (IC50 = 2 μM). In the presence of GABA, loreclezole, an anti-epileptic agent and GABAAR potentiator, binds with highest affinity to the α+-β- site (IC50 = 1 µM), intermediate affinity to the β+-α- sites (IC50 = 10 µM), and low affinity to the γ+-β- site (IC50 = 230 µM). This photolabeling assay was also used to identify allosteric interactions between GABAAR binding sites. Picrotoxinin, a GABAAR channel blocker, is a potent allosteric modulator of R-mTFD-MPAB binding in the presence of bicuculline (IC50 = 0.5 μM), but it is >200-fold weaker (IC50 = 120 μM) in the presence of GABA and has no effect on azietomidate binding. Supported by GM58448.