The oxytocin receptor was first identified using a pharmacological ligandbinding assay in the rat myometrium (Soloff and Swartz 1973). Its peptide ligand, oxytocin, belongs to the nonapeptide hormone family comprising both oxytocin-like (mesotocin, isotocin, etc.) and vasopressin-like (vasotocin, phenypressin, etc.) cyclic peptides. Amongst eutherian mammals, arginine8-vasopressin (AVP), though having its own specific receptors, was shown to bind to the oxytocin receptor with almost as high affinity as oxytocin itself. Oxytocin was the first peptide hormone whose amino acid sequence was completely elucidated, and the first to be chemically synthesized (Du Vigneaud et al. 1953a, b). Because of the relatively simple structure of these peptide hormones, a large number of synthetic agonists and antagonists have been developed and tested (reviewed by Manning et al. 1995). Pharmacological studies using such peptides did allow a degree of speculation on structural aspects of the receptor-ligand binding interaction, though only upon the cloning of the receptor some 40 years after the elaboration of the peptide hormone has it been possible to pursue such studies in detail.