The effect of the general anesthetic propofol on t-[35S]butylbicyclophosphorothionate ([35S]TBPS) binding to unwashed membrane preparations from rat cerebral cortex was studied and compared to that of other general anesthetics (pentobarbital, alphaxalone) which are known to enhance GABAergic transmission. Propofol produced a concentration-dependent complete inhibition of [35S]TBPS binding, an effect similar to that induced by pentobarbital and alphaxalone, although these agents differ markedly in potency (alphaxalone greater than propofol greater than pentobarbital). The concomitant addition of propofol either with alphaxalone or pentobarbital produced an additive inhibition of [35S]TBPS binding, suggesting separate sites of action or different mechanisms of these drugs. Moreover, although bicuculline (0.1 microM) completely antagonized the propofol-induced inhibition of [35S]TBPS binding, the effect of this anesthetic was not due to a direct interaction with the gamma-aminobutyric acidA (GABAA) recognition site. In fact, propofol, like alphaxalone and pentobarbital, markedly enhanced [3H]GABA binding in the rat cerebral cortex. Finally, propofol was able to enhance [3H]GABA binding in membranes previously incubated with the specific chloride channel blocker picrotoxin. Taken together these data strongly suggest that propofol, like other anesthetics and positive modulators of GABAergic transmission, might exert its pharmacological effects by enhancing the function of the GABA-activated chloride channel.