Binding of [3H]CGS 21680, an agonist radioligand selective for A2-adenosine receptors (A2AR), to membranes and solubilized preparations from bovine brain striatum revealed labelling of a single high affinity binding state. In membranes, guanine nucleotides per se were ineffective in modulating agonist binding whereas cations, Na+ and Mg++, had distinct effects. The addition of NaCl (200 mM) as well as the Mg(++)-free preparation of membranes led to a significant decrease in binding affinity and the number of binding sites. Moreover, the presence of Na+ was required for the demonstration of a guanine nucleotide effect, i.e. a decrease in maximal binding. Following solubilization, agonist-A2AR interactions were sensitive to guanine nucleotides even in the absence of Na+; guanine nucleotides and Na+ had additive effects in reducing the number of binding sites. Moreover, the effect of GTP was reversible, i.e. binding returned to control levels upon removal of the nucleotide. This suggests the A2AR and its G protein (presumably Gs) are solubilized as a functional unit and may not dissociate even in the presence of GTP following solubilization. We, therefore, believe that a "tight" association exists between receptor and G protein (Gs), and that guanine nucleotides and sodium act at different sites on the R-G complex. Drawing an analogy with similar observations on the avian beta-adrenergic receptor (Hertel et al, J. Biol. Chem. 265:17988-94, 1990; Parker & Ross, J. Biol. Chem. 266:9987-96, 1991) we postulate that the regulatory features of the A2AR can be attributed to a distinct receptor domain that interacts with cellular regulatory elements.