In equilibrium binding studies, using 3H-etorphine and 3H-diprenorphine, digitonin extracts of frog brain membranes are found to contain two classes of sites, one of which is seen only in the presence of Na+ ions. Centrifugation of the extracts in sucrose gradients separates two macromolecular components (10S and 12S) which display specific opiate binding activity. The 12S component appears to carry the site that binds opiates in the absence of Na+ ions while the 10S component would carry the other site, i.e. the one which is seen only in the presence of Na+ ions in equilibrium binding studies. Preliminary evidence is also given that in extracts of frog brain membranes which have been pre-incubated with 120 mM NaCl, the balance of the two components is shifted in favor of the slower sedimenting (10S) one. These results are discussed in terms of the regulation of the state of equilibrium between an agonist (12S) and an antagonist (10S) form of the opioid receptor.