GR231118 TFA, an analogue of the C-terminus of neuropeptide Y, is a potent , competitive and relative seletive antagonist at human neuropeptide YY receptor with a pK i of 10.4. GR231118 a potent agonist at the human neuropeptide YY4 receptor ( pEC 50 =8.6; pK i =9.6) and a weak agonist at the human and rat neuropeptide Y Y2 and Y5 receptors. GR231118 also has high affinity for the mouse neuropeptide YY6 receptor ( pK i = 8.8).