Dopamine exerts its action by binding to five distinct dopamine receptors, all of which belong to G protein-coupled receptor family (Missale et al. 1998). The D1 subtype is the most abundant dopamine receptor in the central nervous system. Activation of D1 receptor stimulates adenylyl cyclase, activates cyclic AMP-dependent protein kinases. It regulates neuronal growth and development, mediate some behavioral responses and modulate dopamine receptor D2-mediated events. Cloned human D1-expressing cell line is made in the Chem-6 host, which supports high levels of recombinant D1 expression and contains optimized levels of a recombinant promiscuous G protein to couple the receptor to the calcium signaling pathway. Thus, the cell line is an ideal tool for screening for agonists, antagonists and modulators at D1.