Detailed displacements of [3H]dihydromorphine by ketocyclazocine and SKF 10,047, [3H]ethylketocyclazocine by SKF 10,047, and [3H]SKF 10,047 by ketocyclazocine are all multiphasic, suggesting multiple binding sites. After treating brain tissue in vitro with naloxazone, all displacements lose the initial inhibition of 3H-ligand binding by low concentrations of unlabeled drugs. Together with Scatchard analysis of saturation experiments, these studies suggest a common site which binds mu-, kappa, and sigma-opiates and enkephalins equally well and with highest affinity (KD less than 1 nM). The ability of unlabeled drugs to displace the low affinity binding of [3H]dihydromorphine (KD = 3 nM), [3H]ethylketocyclazocine (KD = 4 nM), [3H]SKF 10,047 (KD = 6 nM), and D- Ala2-D-Leu5-[3H]enkephalin (KD = 5 nM) remaining after treating tissue with naloxazone demonstrates unique pharmacological profiles for each. These results suggest the existence of distinct binding sites for kappa- and sigma-opiates which differ from those sites which selectively bind morphine (mu) and enkephalin (delta).