The relative affinities of the optical isomers of methadone, .alpha.-methadol, .alpha.-acetylmethadol and their N-demethylated derivatives in competing for both 3H-DHM [dihydromorphine] and 3H-NLX [naloxone] binding [using rat brain homogenate] paralleled their analgesic effects. l-Methadone was 30 times as effective as d-methadone in competing for 3H-DHM and 3H-NLX binding sites. The reduction of l-methadone to .alpha.-d-methadol and subsequent N-demethylation to .alpha.-d-normethadol reduced its effectiveness as indicated by the increase in the IC50 [median inhibitory concentration dosage] values for 3H-DHM and 3H-NLX binding. The reduction of d-methadone followed by N-demthylation produced a potent derivative, .alpha.-l-normethadol, which had IC50 values on 3H-DHM and 3H-NLX binding similar to those of l-methadone. The affinity of .alpha.-l-acetylmethadol on the binding of both 3H-ligands was between those of l-methadone and d-methadone, and increased as it was N-demethylated. .alpha.-d-Acetylmethadol was more effective than .alpha.-l-acetylmethadol in competing for both 3H-ligands from the opiate receptors, and its affinity, unlike that of .alpha.-l-acetylmethadol, decreased when it was N-demethylated. The affinities of the methadone isomers and related compounds on the binding of 3H-NLX fell in the presence of Na+.