Neuropeptide Y (NPY) is widely distributed throughout the central and peripheral nervous system and exerts a wide range of physiological responses by activating specific receptors. In this study we have characterized the potency of the high affinity peptide dimer antagonist, GR231118, to displace radiolabeled NPY/PYY from different tissues and cell lines expressing Y1 or Y2 receptors and from CHO cells stably transfected with human cDNA encoding for Y1, Y2 and Y4 receptors. GR231118 displays high affinity for Y1 and Y4 receptors, equal or better than that of NPY itself, while its activity is several fold weaker for Y2 receptors. Displacement of radiolabeled PYY from rat hypothalamic membranes by GR231118, reveals the existence of high and low affinity binding sites which may be equated to Y1 and Y2 receptors respectively suggesting that the compound maybe used as a tool to dissect central NPY receptors.