Recently [3H]-CGS 21680 (2-[p-(2-carbonyl-ethyl)-phenylethylamino]- 5'-N-ethylcarboxamidoadenosine) has been identified as a selective adenosine A2-receptor agonist. In this study the binding of [3H]-CGS 21680 to 10 microns sections of rat neostriatum was investigated with quantitative autoradiography. Specific, saturable binding was detectable, and Scatchard analysis of saturation experiments gave estimates for KD and Bmax of 1.7 nM and 322 fmol/mg protein, respectively. The rank order of potency for inhibition of [3H]-CGS 21680 binding was 5'-N-ethylcarboxamidoadenosine (1.9 nM) greater than 2-chloroadenosine (18 nM) greater than R-N6-phenylisopropyladenosine (59 nM) greater than S-N6-phenylisopropyladenosine (460 nM) greater than 1,3-dipropyl-8-cyclopentylxanthine (700 nM). The binding of [3H]-CGS 21680 was sensitive to GTP, since 1 microM GTP reduced binding to 4.7% of control. These data support the identity of CGS 21680 as an agonist at high affinity adenosine A2-receptors and indicate these receptors in rat striatum are coupled to guanine nucleotide binding proteins.