5-Hydroxytryptamine (5-HT, also commonly known as serotonin) is synthesized in enterochromaffin cells in the intestine and in serotonergic nerve terminals. In the periphery, 5-HT mediates gastrointestinal motility, platelet aggregation, and contraction of blood vessels. Many functions of the central nervous system are influenced by 5-HT, including sleep, motor activity, sensory perception, arousal and appetite. A family of 12 GPCRs and one ion channel mediate the biological effects of 5-HT (Hoyer et al., 1994). 5-HT2C, which couples to Gq in most cells to stimulate intracellular calcium, is prominently expressed in brain and appears to modulate depression, anxiety and appetite (Miller, 2005; Serretti et al., 2004; Wood, 2003). The mRNA encoding 5-HT2C undergoes selective RNA editing that changes 4 amino acids in the second intracellular loop; these changes result in alteration of efficiency of coupling to G proteins. Alterations in editing of 5-HT2C have been detected in victims of suicidal depression and in mice treated with the SSRI, fluoxetine (Tohda et al., 2006). The cloned human 5-HT2C -expressing cell line is made in the Chem-1 host, which supports high levels of recombinant 5-HT2C 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 5-HT2C and its ligands.