The selectivities of morphine, codeine, l-methadone and d-propoxyphene, towards the binding sites in mouse brain membranes labelled by 3H-dihydromorphine (DHM), 3H-ethylketocyclazocine (EKC) and 3H-D-Ala2-Leu5-enkephalin (DALE), were investigated. Of the four binding sites identified, three correspond to mu-, kappa- and delta-opioid binding sites or receptors, respectively. The fourth site has a high capacity and binds EKC with a high affinity, DHM with a very low affinity and does not bind DALE. In displacement studies, the relative affinities of morphine and methadone were quite similar towards the tree sites with highest affinity (mu much greater than kappa much greater than delta). Codeine and d-propoxyphene were mu-selective but did not differentiate between kappa- and delta-sites. At high concentrations I-methadone (Kd=6.7 microM), and d-propoxyphene (Kd=40 microM) bound to the fourth site, while morphine, codeine and naloxone were practically inactive. The binding selectivities of these drugs were quite different from those of metkephamid and U-50, 488 H, substances that are thought to exert their antinociceptive effects through delta- and kappa-receptors, respectively. It was concluded that while d-propoxyphene and codeine may partly act through other receptors than morphine, this is probably not the case for l-methadone.