Previous In vivo studies revealed that the mixed agonist-antagonist buprenorphine can down-regulate μ and up-regulate δ2 and κ1, opioid receptors in rat brain. In this report brain regional differences in opioid receptor adaptation were addressed. Rats received i.p. injections with buprenorphine (0.5–2.5 mg/kg) and were killed 20 h later. Membranes from 7 brain regions were analyzed for μ (3H-[D-Ala2,N-mephe4,Gly-ol5] enkephalin), κ1, (3H-U-69593), δ1(3H-[D-Pen2,D-Pen5] enkephalin) and δ2 (3H-deltorphin II) receptor binding parameters. Buprenorphine induced down-regulation of μ receptors in frontal cortex, occipital cortex, thalamus, hippocampus, striatum and brain stem. Kd values for 3H-[D-Ala2,N-mephe4,Gly-ol5] enkephalin were unchanged from controls. Up-regulation of κ1, receptors was observed in frontal, parietal, occipital cortexes and striatum. Binding to δ2 sites was elevated in frontal and parietal cortexes. Buprenorphine did not alter δ1 binding in any of the regions examined. Changes in opioid receptor adaptation induced by buprenorphine were further supported by data from cross-linking of 125l-β-endorphin to cortical membrane preparations. A reduction in a 60- to 65-kDa band was detected in frontal and occipital cortices in which binding assays revealed down-regulation of μ receptors. In parietal cortex neither the 60- to 65-kDa product nor Bmax changes were observed. These results indicate that buprenorphine is a useful tool to study brain opioid receptor adaptation in vivo and the information accrued may be relevant to the mode of action of this drug in the treatment of heroin and cocaine abuse.