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 5-HT2C membrane preparations are crude membrane preparations made from our proprietary stable recombinant cell lines to ensure high-level of GPCR surface expression; thus, they are ideal HTS tools for screening of antagonists of 5-HT2C interactions and its ligands. The membrane preparations exhibit a Kd of 5.9 nM for [3H]-mesulergine. With 6 nM [3H]- Mesulergine, 10 mg/well 5-HT2C Membrane Prep yields greater than 5 fold signal-to-background ratio.