We present evidence that stimulation of the human beta-3 adrenergic receptor (AR), expressed in Chinese hamster ovary/K1 cells, specifically activates the mitogen-activated protein kinases extracellular signal-regulated kinase (ERK)1 and 2, but not JNK or p38. The extent and kinetics of the ERK stimulation by the beta-3 AR are identical with those of the endogenic insulin receptor. However, insulin augments cellular proliferation, whereas beta-3 AR agonists inhibit proliferation due to the production of cyclic AMP. The pharmacological profile of the ERK activation by the beta-3 AR differs significantly from its activation of adenylyl cyclase. The order of potency and intrinsic activities of both natural ligands, norepinephrine and epinephrine, is inversed between both signaling pathways. In addition, BRL 37344 and propranolol, ligands that act as agonists in the stimulation of cyclase, act as antagonists for ERK activation. The activation of ERK1/2 is sensitive to pertussis toxin, suggesting that the beta-3 AR, in addition to its interaction with Gs, can couple to Gi/o. Furthermore, the activation of ERK by the beta-3 AR is sensitive to PD98059, wortmannin, and LY294002, indicating a crucial role for mitogen-activated protein kinase kinase and phosphatidylinositol-3 kinase (PI3K), respectively. A beta-3 AR-mediated stimulation of PI3K is confirmed by the observation that the selective agonist CGP 12177A specifically activates protein kinase B. As was observed for the activation of ERK, the activation of protein kinase B is inhibited by preincubation with pertussis toxin and PI3K inhibitors, suggesting that both are a consequence of a Gi/o-mediated activation of PI3K.