Benzodiazepine (BZ) recognition sites of the 'peripheral' type were localized autoradiographically in human postmortem brain and kidney using [3H]Ro 5-4864. These sites presented a relatively homogeneous distribution. Areas such as the ependyma, choroid plexus and olfactory bulb, which in the rat are very rich in these binding sites, presented densities in the human brain which were about 1/10 of those seen in the rat. Human tissues presenting gliosis, such as the hippocampi from senile dementia patients, did not show a clear increase in the number of [3H]Ro 5-4864 sites, in contrast with the high densities found in rat brain areas presenting neurotoxin-induced gliosis. Intermediate densities of binding were seen in a human glioblastoma tumor. The human kidney also showed lower densities of peripheral BZ binding sites, when compared to the rat kidney. These results indicate that marked species differences exist in the densities of peripheral BZ sites and that caution has to be exerted when extrapolating data from the experimental animal to human.