With the finding that brown adipose tissue is present and negatively correlated to obesity in adult man, finding the mechanism(s) of how to activate brown adipose tissue in humans could be important in combating obesity, type 2 diabetes, and their complications. In mice, the main regulator of nonshivering thermogenesis in brown adipose tissue is norepinephrine acting predominantly via β₃-adrenergic receptors. However, vast majorities of β₃-adrenergic agonists have so far not been able to stimulate human β₃-adrenergic receptors or brown adipose tissue activity, and it was postulated that human brown adipose tissue could be regulated instead by β₁-adrenergic receptors. Therefore, we have investigated the signaling pathways, specifically pathways to nonshivering thermogenesis, in mice lacking β₃-adrenergic receptors. Wild-type and β₃-knockout mice were either exposed to acute cold (up to 12 h) or acclimated for 7 wk to cold, and parameters related to metabolism and brown adipose tissue function were investigated. β₃-knockout mice were able to survive both acute and prolonged cold exposure due to activation of β₁-adrenergic receptors. Thus, in the absence of β₃-adrenergic receptors, β₁-adrenergic receptors are effectively able to signal via cAMP to elicit cAMP-mediated responses and to recruit and activate brown adipose tissue. In addition, we found that in human multipotent adipose-derived stem cells differentiated into functional brown adipocytes, activation of either β₁-adrenergic receptors or β₃-adrenergic receptors was able to increase UCP1 mRNA and protein levels. Thus, in humans, β₁-adrenergic receptors could play an important role in regulating nonshivering thermogenesis.