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-adrenoceptors (Bylund et al., 1994). The three members of the α1 subclass of adrenoceptors, α1A, α1B and α1D, couple to Gq, and promote contraction of vascular and urinary tract smooth muscle, relaxation of intestinal smooth muscle, increased contractile force in the heart, and glycogenolysis and gluconeogenesis in the liver. The different subtypes have overlapping distributions and variably contribute to these effects depending on species and tissue. Overexpression of a constitutively active α1B mutant in the heart of transgenic mice resulted in cardiac hypertrophy with increased heart weight/body weight ratios. Analysis of α1B knock out mice has provided evidence that α1B is a mediator of blood pressure and aortic contractile responses induced by α1 agonists (Milano et al., 1994). The locomotor and rewarding effects of pysochostimulants and opiates were suppressed in mice lacking α1B-adrenergic receptors (Drouin et al. 2002). Millipore's α1B 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 α1B. The membrane preparations exhibit a Kd of 0.8 nM for [3H]-Prazosin. With 1 nM [3H]-Prazosin, 5μg/well α1B Membrane Prep typically yields greater than 5-fold signal-to-background ratio.