The catecholamines epinephrine and norepinephrine produce a wide variety of physiological responses in target cells. Some of these are mediated by β-adrenergic receptors (β-ARs) through the stimulation of adenylyl cyclase and the production of cyclic AMP as first demonstrated by Sutherland and his colleagues (1). Although β-ARs were initially divided into β₁-AR (norepinephrine ≥epinephrine) and β₂-AR (epinephrine > norepinephrine) subtypes by Lands et al. (2), a third subtype, the β₃-AR, has now been recognized. The latter was first identified in adipocytes based on atypical responses to antagonists and later to novel agonists (reviewed in ref. 3). The three subtypes, from both human and rat, have been cloned, and although they share limited homology in amino acid sequences, they all are members of the seven membrane-spanning rhodopsin superfamily, and couple to the stimulatory G-protein and activate adenylyl cyclase.