C-6 derivatives--hydrazones, phenylhydrazones, dinitrophenylhydrazones, oximes and semicarbazones--of morphinane-6-ones were synthesized and their binding characteristics were studied on rat brain membranes. The dihydromorphinone and oxymorphone derivatives compete for the (3H)naloxone binding sites with high affinity, while the dihydrocodeinone and oxycodone derivatives are less potent. The affinity of the new compounds is decreased for the delta sites as compared to the parent ligands. The ligands bearing bulky substituents also bind with low affinity to the kappa sites. The modification decreased the Na(+)-index of compounds indicating their mixed agonist-antagonist character. The dihydromorphinone derivatives are all capable to block irreversibly the high affinity binding site of (3H)naloxone, whereas the dihydrocodeionone derivatives block irreversibly the low affinity site. A possible mechanism for the inhibition is suggested.