We previously reported that GABA-evoked currents of rat retinal ganglion cells were modulated by redox agents. In this study, we further characterized the effects of redox modulation on GABA receptors using recombinant human subunits in the Xenopus oocyte expression system with two-electrode voltage-clamp recording. GABA receptors composed of subunits α 1–3, β 1–3, γ 1, γ 2S, and ρ 1 were expressed. The sulfhydryl reducing agent dithiothreitol reversibly potentiated the responses of various combinations of functional recombinant GABA A subunits, whether expressed as triplets (α 1β 1–3γ 1,2S), pairs (α 1–3β 1–3; β 1–3γ 1,2S), or singly (β 2). These effects of dithiothreitol were rapidly reversible, and the oxidizing agent 5-5′-dithiobis-2-nitrobenzoic acid exerted the opposite effect. In contrast to these effects on GABA A receptors, dithiothreitol had no effect on the responses of homomeric GABA ρ 1 (GABA C) receptors. The degree of dithiothreitol potentiation of GABA A receptor responses depended on subunit composition. Co-expression of γ 2S with α 1β 1–3 subunits resulted in markedly less dithiothreitol potentiation of GABA-evoked currents than that observed for α 1–3β 1–3 subunits in the absence of γ 2S. None the less, the magnitude of dithiothreitol potentiation could be restored by using a combination of lower GABA concentrations (5–10 μM) and higher dithiothreitol concentrations (5–20 mM). N, N, N′, N′-tetrakis(2-pyridyl-methyl)ethylenediamine, a high-affinity Zn 2+ chelator, also potentiated GABA A receptor currents. However, the potentiation produced by 10 mM dithiothreitol was larger than that produced by saturating concentrations of N, N, N′, N′-tetrakis(2-pyridyl-methyl)ethylenediamine (100 μM), implying that at least part of the effect of dithiothreitol was due to redox modulation rather than Zn 2+ chelation. Dithiothreitol also potentiated the spontaneous current of homomeric GABA A receptors composed of β subunits. Mutation of a single cysteine residue in the M3 domain, yielding homomeric β 3(C313A) receptors, abrogated dithiothreitol potentiation of the spontaneous current. In summary, this study further characterizes the modulatory effects of redox agents on recombinant GABA A receptors. The degree of redox modulation of GABA A receptors depended on subunit composition. In contrast to their effect on GABA A receptors, redox agents were not found to modulate GABA C receptors composed of homomeric ρ 1 subunits. Using site-directed mutagenesis, a cysteine residue was located in the β 3 subunit which may comprise one of the redox-active sites that underlies the modulation of heteromeric GABA A receptors by reducing and oxidizing agents.