The regulation of mu- and delta-opiate receptors by guanine nucleotides and cations was studied by examining the binding of [3H][DAla2, DLeu5]enkephalin to rat brain membranes. The binding to mu-opiate receptors could be suppressed by 1 microM [DPro4]morphiceptin, a highly specific mu-agonist, thus permitting separate assessment of delta-opiate receptor binding. GTP, GDP, and the nonhydrolyzable analogs 5'-guanylyl imidodiphosphate (Gpp[NH]p) and guanosine 5'-O-(2-thiodiphosphate) (GDP-S) effectively decreased the binding to both receptor types. This inhibitory effect was potentiated by Na+. The inhibitory effect of GTP and GDP, but not of the nonhydrolyzable analogs, was reversed by Mg2+. Pretreatment of membranes with GDP or GTP increased substantially the subsequently measured high-affinity binding, and this effect required the presence of Mg2+ in the binding assay. Similar pretreatment with GDP-S resulted in only a partial increase of binding compared to GTP or GDP, and Gpp[NH]p was relatively ineffective. Similar results were observed for both mu- and delta-receptors, although the effects on mu-receptors were quantitatively more profound. These data suggest that guanine nucleotides play a dual function in regulating opiate receptor binding in a manner dependent upon the presence of Na+ or Mg2+.