Apelin peptides have been discovered to be a family of peptides of different sizes that is derived from the N-terminus of a 77 amino acid precursor peptide (preproapelin) (Hosoya et al., 2000). Apelin receptor (APJ) is a G protein-coupled receptor that is activated by several apelin fragments. APJ activation leads to the inhibition of cAMP production. APJ and apelin peptides have been found to be involved in the regulation of cardiovascular function (Katugampola et al., 2001) and fluid homeostasis (Reaux et al., 2001). Broad roles of apelin system has been established in lowering blood pressure, as a potent cardiac inotrope, in modulating pituitary hormone release and food and water intake, in stress activation, and as a novel adipokine that is excreted from fat cells and regulates insulin (Lee et al., 2006) . APJ 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 APJ. The membrane preparations exhibit a Kd of 0.4 nM for [125I]-Apelin. With 0.25 nM [125I]-Apelin, 5 mg/well APJ Membrane Prep typically yields greater than 20-fold signal-to-background ratio.