The benzodiazepine receptor was localized in rat brain with light microscopic autoradiographic methods. The binding of [3H]flunitrazepam to slide-mounted tissue sections had all of the characteristics associated with the benzodiazepine receptor. It was saturable, or a high affinity, affected by chloride ion and gamma-aminobutyric acid and showed an appropriate pharmacology. Autoradiograms were generated by the apposition of emulsion-coated coverslips. Receptor distribution showed striking variation throughout the brain and spinal cord. Some areas showing high densities include the cerebral cortex, the molecular layer of the cerebellum, parts of the limbic system, olfactory bulb and hypothalamus and substantiae gelatinosae of the spinal trigeminal nucleus and spinal cord. White matter areas showed negligible levels of receptor. These localizations show which parts of the brain are affected by benzodiazepine administration and contribute to our understanding of the mechanism of the well-known properties of the drugs such as their anxiolytic and anticonvulsant effects.