Dopamine is a catecholamine neurotransmitter that functions in the CNS to control locomotor, cognitive, emotional and neurendocrine processes, and in the periphery to modulate cardiovascular, renal and gastrointestinal processes. The biological activities of dopamine are mediated by a family of five GPCRs. The D1 and D5 subtypes couple to Gs to increase intracellular cAMP, whereas the D2, D3 and D4 subtypes couple to Gi to reduce cAMP (Missale et al., 1998). The hypertensive phenotype of mice with a targeted deletion of D5 indicates that D5 regulates central control of sympathetic vascular tone (Hollon et al., 2002). In addition, D5 modulates locomotion and corticostriatal long-term depression (Centonze et al., 2003). The cloned human D5-expressing cell line is made in the Chem-1 host, which supports high levels of recombinant D5 expression on the cell surface and contains high levels of the promiscuous G protein Ga15 to couple the receptor to the calcium signaling pathway. Thus, the cell line is an ideal tool for screening for antagonists of interactions between D5 and its ligands. The membrane preparations exhibit a Kd of 2.25 nM for [3H]-SCH 23390. With 0.5 nM [3H]-SCH 23390, 10 mg/well D5 Membrane Prep yields greater than 3-fold signal-to-background ratio.