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 alpha- and beta-adrenoceptors. The three members of the alpha1 subclass of adrenoceptors, alpha1A, alpha1B and alpha1D, 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 alpha1B mutant in the heart of transgenic mice resulted in cardiac hypertrophy with increased heart weight/body weight ratios. Analysis of alpha1B knock out mice has provided evidence that alpha1B is a mediator of blood pressure and aortic contractile responses induced by alpha1 agonists. The locomotor and rewarding effects of pysochostimulants and opiates were suppressed in mice lacking alpha1B-adrenergic receptors.