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. D1 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 H1 interactions and its ligands. The membrane preparations exhibit a Kd of 1.5 nM for [3H]-SCH 23390. With 1.5 nM [3H]- SCH 23390, 2.5 mg/well and 10 mg/well D1 Membrane Prep typically yield greater than 5 and 15-fold signal-to-background ratios, respectively.