The presence, density and distribution of 'peripheral' benzodiazepine (BZ)-binding sites was investigated by autoradiography in the brains of rats, mice, guinea pigs and cats and in some areas of the dog and monkey brains, using [3H]Ro 5-4864 as a ligand. Marked interspecies differences were found in the distribution and densities of these sites. Rats and mice presented a low density of binding uniformly distributed throughout the brain with high densities concentrated in the ependyma, choroid plexus and olfactory nerve layer of the olfactory bulb. In contrast, guinea pig and cat brains presented relatively high concentrations of peripheral BZ binding sites throughout the grey matter ependyma and choroid plexus, but low in the olfactory bulb. Monkey and dog brains presented low densities of peripheral BZ binding sites, including the choroid plexus.