The aim of this thesis was to investigate how the spatial arrangements of amino acids in both the ligand and influence gonadotrophin-releasing hormone (GnRH) ligand-receptor interactions. A number of residues in the receptor extracellular loops (ECLs) have been identified as interacting directly with ligands. Therefore, these regions play a crucial role in ligand binding. Chimeric receptors, consisting of the human receptor containing the three catfish receptor ECLs in all single, double and triple combinations were produced. Certain combinations of ECLs were found to have different effects on the binding of mGnRH, [D-Trp⁶]-GnRH, GnRH II, [D-Lys⁶]-GnRH II and Antagonist 135-18. ECL3 appears to influence the binding of mGnRH, but not [D-Trp⁶]-GnRH, which correlates with previous studies. The difference in the binding affinity of these two ligands could be attributed to the ECLs, as their affinities at the triple-ECL substituted chimeric receptor were not significantly different to those at the wild type catfish receptor. Substitution of the ECLs did not simulate binding at the wild type catfish receptor for GnRH II, [D-Lys⁶]-GnRH II or Antagonist 135-18. This implies that these ligands form different interactions with GnRH receptors compared with those formed by mGnRH and [D-Trp⁶]-GnRH. Potential interactions between ECLs were investigated with additional mutant receptors. ECL interactions may influence ligand binding by altering the spatial arrangement or accessibility of ligand contact sites. Interactions between ECLs 2 and 3 appear to be particularly important: they are partly responsible for differences in the binding of GnRH II at the catfish compared with at the human receptor; a single residue in ECL2, namely Glu^5.35, is likely to be responsible for the different affinity of GnRH II at the rat receptor compared with at the mouse and human receptors as a result of repulsion between ECLs 2 and 3; and human and chicken GnRH receptors appear to have important differences in ECLs 2 and 3 such that agonist binding at human-chicken chimeric receptors is severely affected.