Activation of adenylyl cyclase by b-adrenergic receptors (bARs) plays a major role in adipose tissue homeostasis. The increase in cAMP promotes lipolysis in white adipose tissue, activates both ther- mogenesis and lipolysis in brown adipose tissue (BAT), and induces BAT hypertrophy. Previous studies indicated that among the three bAR subtypes present in adipose tissue, b3AR could be a potential target for antiobesity treatments in humans. We studied immortal- ized human brown adipocytes (PAZ6 adipocytes) as a model of b-ad- renergic response in human BAT. PAZ6 adipocytes and freshly iso- lated mature human brown adipocytes display the same proportions of bAR subtypes, with b3AR being the most abundant (;80% of the total). However, b3AR was poorly coupled to the adenylyl cyclase pathway in PAZ6 cells, contributing to only 10% of the isoproterenol- induced accumulation of cAMP, whereas 20% and 70% of the signal depended on b1- and b2-subtypes, respectively. Upon isoproterenol stimulation, b1- and b2AR down-regulated with a half-life of about 3 h and the b3AR with a half-life of 30 - 40 h. Long term stimulation with both saturating (micromolar) and nonsaturating (nanomolar) con- centrations of b-adrenergic agonists caused a complete desensitiza- tion of the b-adrenergic response at the adenylyl cyclase level and loss of stimulated protein kinase A activity and CREB phosphorylation. These results suggest that cAMP-dependent processes will be desen- sitized upon permanent treatment with b3AR agonists. Further stud- ies should establish whether the b3AR is coupled to other signaling pathways in human brown adipocytes and whether these may con- tribute to BAT hypertrophy and/or thermogenesis. (Endocrinology 139: 2676 -2684, 1998)