The effects of guanosine-5'-triphosphate (GTP), sodium chloride and manganese chloride on the binding of buprenorphine to opiate receptors present in rat brain has been studied. Manganese chloride significantly decreased the affinity of binding of both [3H] buprenorphine and unlabelled buprenorphine to morphine and enkephalin receptors. Guanosine-5'-triphosphate increased the affinity of buprenorphine for morphine sites, but had no effect on binding of buprenorphine to enkephalin or benzomorphan sites, or binding of [3H]buprenorphine. Sodium chloride had no effect on binding of buprenorphine. Control studies indicated similar apparent affinities of buprenorphine for morphine (Ki = 0.30 nM) and enkephalin (Ki = 0.31 nM) sites, and lower affinity for benzomorphan sites (Ki = 4.16 nM). No evidence could be obtained for a differential effect of ions or guanosine-5'-triphosphate on binding of buprenorphine to opiate receptor subtypes.