Arginine vasopressin (AVP) is a 9 amino acid peptide that functions as an antidiuretic, vasoconstrictor and neurotransmitter. The three vasopressin receptors, V1A, V1B and V2, are GPCRs; V1A and V1B couple to Gq and calcium release, whereas V2 couples to Gs (Birnbaumer, 2000). The V1B receptor is expressed prominently in the anterior pituitary, where it mediates vasopressin-induced release of ACTH (Tanoue et al., 2004). A selective antagonist of V1B has recently been developed and shown to reduce depression, anxiety, and aggression in rodents (Blanchard et al, 2005; Griebel et al., 2002). The cloned human V1B-expressing cell line is made in the Chem-1 host, which supports high levels of recombinant V1B expression on the cell surface and contains high levels of the promiscuous G protein Gα15 to couple the receptor to the calcium signaling pathway. Thus, the cell line is an ideal tool for screening for antagonists of interactions between V1B and its ligands.