1. The receptor for glucagon-like peptide-1 (GLP-1) can be activated by both its physiological hormone and a peptide discovered in the venom of the Gila Monster, exendin-4, which shows promise as an antidiabetic agent. 2. Exendin-4 displays receptor-binding properties not observed for GLP-1. Firstly, exendin-4 can be truncated by up to eight residues at its N-terminus without a significant loss of affinity. Secondly, exendin-4 maintains high affinity for the isolated N-terminal domain of the receptor, suggesting that exendin-4 makes additional contacts with this domain of the receptor, which nullify the requirement for ligand-receptor interactions involving the extracellular loops and/or transmembrane helices of the receptor's core domain. 3. In order to further understand the nature of the receptor-peptide interaction, a variety of full length and truncated peptide analogues were used to quantify the contribution of each distinct region of exendin-4 and GLP-1 to receptor affinity. 4. Our data show that, for both exendin-4 and GLP-1, the primary interaction is between the putative helical region of the peptide and the extracellular N-terminal domain of the receptor. 5. However, we demonstrate that the contribution to receptor affinity provided by the N-terminal segment of GLP-1 is greater than that of exendin-4, while the C-terminal nine residue extension of exendin-4, absent in GLP-1, forms a compensatory interaction with the N-terminal domain of the receptor. 6. We describe a peptide-receptor binding model to account for these data.