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 cloned human α1B-expressing cell line is made in the Chem-1 host, which supports high levels of recombinant α1B expression on the cell surface and contains high levels of the promiscuous G protein to couple the receptor to the calcium signaling pathway. Thus, the cell line is an ideal tool for screening for antagonists of interactions between α1B and its ligands.