The endogenous catecholamines epinephrine and norepinephrine have profound effects on smooth muscle activity, cardiac function, carbohydrate and fat metabolism, hormone secretion, neurotransmitter release, and central nervous system actions. These activities are mediated by GPCRs belonging to two subfamilies, the α- and β-adrenergic receptors (Bylund et al., 1994). The three members of the β-adrenergic receptor family, β1, β2 and β3, couple to Gs to increase cAMP upon activation. In the heart, the β1 receptor constitutes 70-80% of the β-adrenergic receptors. Activation of cardiac β-adrenergic receptors acutely increases heart rate, cardiac output, and cardiac automaticity, and chronically increases cardiac myocyte apoptosis. In failing hearts, the β1 subtype is downregulated and desensitized, probably as a result of increased catecholamine levels. As a result, β-adrenergic receptor antagonists (β blockers) are effective in the treatment of congestive heart failure and arrhythmia (Lohse et al., 2003). Millipore's β1 adrenoceptor 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 β1 adrenoceptor interactions. The membrane preparations exhibit a Kd of 2.6 nM for [125I]-(-) Iodocyanopindolol (ICYP). With 5 µg/well β1 Adrenoceptor Membrane Prep and 0.25 nM [125I]-(-)ICYP, a greater than 15-fold signal-to-background ratio was obtained.