The inhalation anesthetic isoflurane stereoselectively modulates ligand binding to the GABAA receptor complex. The (+)-isomer of isoflurane was more potent and efficacious than the (-)-isomer in enhancing [3H]flunitrazepam binding to benzodiazepine receptors. For example, concentration effect curves for Cl- enhancement of [3H]flunitrazepam binding were significantly different (P < 0.001) in the presence of (+)- and (-)-isoflurane (0.44 and 0.88 mM). At the higher anesthetic concentration, they potency of Cl- to increase [3H]flunitrazepam binding was increased 3.2- and 1.45-fold by (+)- and (-)-isoflurane, respectively (P < 0.05). Likewise, concentration-effect curves for (+) isoflurane-enhanced [3H]flunitrazepam binding were significantly different (P < 0.05-P < 0.001) from the (-)-isomer in the presence of 0-200 mM Cl-. Stereoselectivity was not observed with respect to the potencies of these enantiomers as inhibitors of [35S]t-butylbicyclophosphorothionate (TBPS) binding to sites within the Cl- ionophore. In this measure, the isomers had similar potencies (P > 0.05), although at higher concentrations (> 0.1 mM) (+)-isoflurane produced significantly more inhibition than (-)-isoflurane. While the absolute potency differences between isomers were modest (< or = 2-fold) and measure dependent, these effects were manifested at clinically relevant concentrations of isoflurane and are consistent with in vivo studies demonstrating (+)-isoflurane is a more effective anesthetic than the (-)-isomer. This is the first demonstration of an inhalation anesthetic producing a stereoselective perturbation of the GABAA receptor complex.