beta-Funaltrexamine (beta-FNA) bound to mouse brain membranes in a reversible and an irreversible (not removed by washing of the membrane) manner, and a portion of each type of binding was opioid-specific. Addition of 100 mM NaCl to the incubating medium enhanced the binding of beta-FNA to membranes. Using membranes preincubated with beta-FNA (1 microM) and then washed three times, the maximum number of binding sites available to [3H]morphine was markedly diminished whereas the affinity of morphine for binding sites was not significantly altered. The binding of [3H]naltrexone was also reduced markedly by beta-FNA pretreatment. In similarly pretreated membranes, the binding of [3H]methionine enkephalin [3H][D-Ala2,D-Leu5]enkephalin (DADLE) or [3H]ethylketazocine was reduced to a smaller extent. Using brain membranes from mice pretreated with a single subcutaneous injection of beta-FNA (100 mg/kg) 48 h prior to use, the binding of [3H]methionine enkephalin was unaffected whereas the number of binding sites available to [3H]morphine was significantly reduced. The inhibition by various ligands of the reversible binding of [3H] beta-FNA resembled the relative ability of the same ligands to inhibit the binding of [3H]ethylketazocine. It was concluded that the irreversible portion of the binding of beta-FNA demonstrates a selectivity for mu over delta binding sites, and that the reversible portion of the binding of beta-FNA demonstrates a selectivity for kappa binding sites over mu or delta binding sites. As such, the binding characteristics of beta-FNA are consistent with its profile in vivo and in isolated tissue studies in vitro.