Mice received i.p. injections of either saline or drug before i.p. administration of [3H]Ro 15-1788 (flumazenil), a selective benzodiazepine (BZD) receptor antagonist. Subjects were decapitated and six brain regions (cortex, cerebellum, striatum, hippocampus, hypothalamus and brain stem) were analyzed for the levels of tritium present in each tissue. Benzyl, ethyl and methyl alcohol enhanced in vivo binding of [3H]Ro 15-1788 in a dose-dependent manner in all brain regions studied with mean ED50 values of 2.4, 34.8 and 38.4% volume/kg body weight, respectively. Lorazepam (18 mg/kg) completely blocked effects of benzyl, ethyl and methyl alcohol to increase [3H]Ro 15-1788 binding. These data indicate that the alcohol-induced increases in [3H]Ro 15-1788 binding occurred at the level of the BZD receptor. In addition, these alcohols did not alter the in vivo binding of [3H]diprenorphine, an opioid receptor ligand, further indicating the selective effect of alcohol on BZD receptors. Administration of lorazepam or alprazolam produced a dose-dependent decrease in the binding of [3H]Ro 15-1788 to brain tissue after administration of either saline or 30% volume/kg ethanol. However, low doses of alprazolam increased binding of [3H]Ro 15-1788 in the absence of ethanol. Together, these results suggest that ethanol produces an increase in the number of BZD binding sites in vivo. These alcohol-induced alterations in BZD receptor binding may mediate, in part, the anxiolytic or sedative properties of ethanol.