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). The 5-HT1B receptor (also known as 5-HT1Db) is expressed presynaptically on serotonergic neurons and postsynaptically on non-serotonergic neurons, and it regulates release of 5-HT and acetylcholine. Several brain regions, including globus pallidus, substantia nigra and dorsal subiculum, highly express HT1B. 5-HT1B has been implicated in the physiology of aggression, depression, migraine, anxiety and reward for drugs of abuse (Sari, 2004). 5-HT1B interacts with p11, which increases cell surface expression and function of 5-HT1B (Svenningsson et al., 2006). 5-HT1B 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 agonists and antagonists of 5-HT1B. The membrane preparations exhibit a Kd of 10.5 nM for [3H]-GR125743. With 7 nM [3H]-GR125743, 10mg/well 5-HT1B Membrane Prep typically yields greater than 8-fold signal-to-background ratio.