Oxytocin is a cyclic 9 amino acid peptide that differs from vasopressin in 2 amino acids. Despite the close similarities in sequence, oxytocin and vasopressin have different biological activities and bind to distinct G protein-coupled receptors. The oxytocin receptor, OT, couples primarily to Gq/11 to mobilize intracellular calcium. In female reproduction, oxytocin promotes uterine contraction and lactation; oxytocin is the most commonly used drug for induction of labor, whereas an oxytocin antagonist, atosiban, is under investigation to suppress preterm labor. Oxytocin/OT interaction in the CNS also plays an important role in stress, male and female sexual response, and sociality (Gimpl and Fahrenholz, 2001). Cloned human OT-expressing cell line is made in the Chem-1 host, which supports high levels of recombinant OT expression on the cell surface for functional detection via the calcium signaling pathway. Thus, the cell line is an ideal tool for screening for agonists, antagonists, and modulators at OT.