A series of C-terminal peptide dimers related to neuropeptide Y have been found to be high affinity and selective ligands for Y receptors. The dimers have non-native connectivity and are challenging for synthesis hindering SAR studies, but we have developed useful methods including the development of heterodimeric peptides and fluorescent ligands. We are studying the mechanism by which such ligands have such high affinity, which remain unclear but clearly have implications for understanding GPCR pharmacology more generally.