Arachidonic acid effect on binding of [3H]naloxone to rat brain membranes were studied at opioid receptor subtype level. Arachidonic acid inhibited opioid receptor binding in a dose-dependent manner, both in the presence and absence of sodium. With blockage experiments it was shown that delta-opioid receptors were modulated by arachidonic acid to a greater extent than that of mu-opioid receptors. On the other hand, there was no significant difference in terms of IC50 values for arachidonic acid inhibition of [3H]naloxone binding at agonist and antagonist configuration of the receptor subtypes.