Barbiturates and the related depressant drugs, etazolate and etomidate, inhibited both the binding of [3H]bicuculline methochloride (BMC) to gamma-aminobutyric acid (GABA) receptor sites and the binding of [3H] beta-carboline-3-carboxylic acid methyl ester (beta CCM) to benzodiazepine receptor sites in mammalian brain. These concentration-dependent effects were chemically specific and stereospecific in a manner correlating with the activity of barbiturates to enhance GABA responses in neurons and to enhance GABA and benzodiazepine receptor agonist binding in vitro. The barbiturate inhibition of [3H]BMC binding involved a decrease in affinity which at high concentrations of barbiturates results in an effective complete loss of detectable binding. The maximal inhibition of [3H] beta CCM binding involved a more modest decrease in affinity (increase in KD from 1.35 to 1.85 nM). The barbiturate inhibitions of both ligands could be reversed by picrotoxin, suggesting an indirect action at previously defined picrotoxin/barbiturate modulatory sites on the GABA-benzodiazepine receptor/chloride ion channel complex.