The effects of chronic treatment with haloperidol on [3H]PK 11195 binding and labelling of the peripheral-type benzodiazepine receptor (PBR) in the rat brain and peripheral tissues were investigated using an in vitro receptor binding technique. The intraperitoneal injection with haloperidol (0.3 or 1.0 mg/kg) for 21 days produced a significant increase in the specific [3H]PK 11195 binding only in the olfactory bulb, but not in the other brain areas or peripheral tissues, while single or subchronic (4 days or 7 days) administration with the drug (0.3 mg/kg, i.p.) failed to increase the binding in the olfactory bulb. Scatchard analysis revealed a significant increase in the maximum number of [3H]PK 11195 binding sites in the olfactory bulb after chronic treatment with haloperidol, showing no change in the affinity of the binding. These results suggest that there is an interaction between PBR and the pharmacological actions of haloperidol in the brain area, and that some compensatory mechanism may be involved in the PBR changes.