Previous studies have suggested that two specific amino acid residues in transmembrane segments 2 and 3 of the GABA(A) receptor alpha 2 subunit, Ser270 and Ala291, are critical for the enhancement of GABA(A) receptor function by inhaled anesthetics. The aim of this study was to determine the effects of amino acid substitutions in alpha 1 beta 2 gamma 2s GABA(A) receptors at alpha 1(S270) and at the homologous beta 2(N265) on receptor gating and anesthetic potentiation of GABA-induced responses. The wild-type and mutant receptors were transiently expressed in HEK 293 cells and GABA-induced currents were recorded using whole-cell voltage clamp. Potentiation of responses to a submaximal concentration of GABA by the anesthetics halothane and isoflurane was also examined. Some of the point mutations caused shifts in the GABA dose-response curve, indicating that the mutations changed the apparent affinity of the receptor for GABA. In receptors mutated at alpha 1(S270), the GABA EC(50) is inversely correlated with the volume of the residue of 270. On the contrary, there was no clear relationship between the physical properties of the amino acid residue at 265 in the beta 2 subunit and either the GABA EC(50) or anesthetic modulation, although mutations at N265 altered both parameters in a quantitative manner. These data are consistent with the results of previous work using other subunit combinations, in confirming that alpha 1(S270) may be involved in channel gating, and also may be important in anesthetic binding; the role of beta 2(N265) is less clear.