The presence of different types of opiate binding sites was investigated with the use of a computerized, weighted, nonlinear least-squares regression program. The experimental data were obtained from four groups. Each of three labeled opiate ligands was displaced using each of the same unlabeled ligands. The resulting nine different ligand combinations of each group were evaluated by use of a curve-fitting program. The four groups consisted of the kappa ligand ethylketocyclazocine, the sigma ligand SKF 10047, and the oripavine derivatives etorphine and diprenorphine, each in conjunction with the delta opiate receptor ligand (D-Ala2,D-Leu5)-enkephalin and the mu opiate receptor ligand dihydromorphine. The binding model which best fitted each of the four groups suggested the existence of three different binding sites in the rat brain homogenate. Two of these sites conform to the previously described mu and delta sites. A third site (R3) displayed high affinity for ethylketocyclazocine, SKF 10047, etorphine, and diprenorphine but very low affinity for dihydromorphine and [D-Ala2,D-Leu5]enkephalin. Naloxone, cyclazocine, and dynorphin-(1--13) had high affinity for R3. Behavioral data support the interpretation that the R3 site may represent a kappa site at which SKF 10047 acts antagonistically.