The effects of arachidonic acid (AA) on binding of [3H]naloxone to the agonist and antagonist configurations of opioid receptors were investigated in rat brain. Equilibrium binding parameters of the agonist and antagonist configurations of the receptors were evaluated from homologue displacement data in the presence or absence of AA. Addition of AA at a concentration of 0.6 mM (1.5 m mole/mg of protein) reduced by 22% and 53% the maximal number of binding sites (Bmax) respectively in the absence or presence of 100 mM NaCl. Binding affinity (KD) was not altered significantly (P<0.05) either in the presence or absence of 100 mM NaCl and AA. We conclude that AA mediated reduction in [3H]naloxone specific binding was chiefly due to a decrease in the number of binding sites.