AbstractA sharp increase in mitochondrial Ca²⁺ marks the activation of brown adipose tissue (BAT) thermogenesis, yet the mechanisms preventing Ca²⁺ deleterious effects are poorly understood. Here, we show that adrenergic stimulation of BAT activates a PKA-dependent mitochondrial Ca²⁺ extrusion via the mitochondrial Na⁺/Ca²⁺ exchanger, NCLX. Adrenergic stimulation of NCLX-null brown adipocytes (BA) induces a profound mitochondrial Ca²⁺ overload and impaired uncoupled respiration. Core body temperature, PET imaging of glucose uptake and VO₂ measurements confirm a thermogenic defect in NCLX-null mice. We show that Ca²⁺ overload induced by adrenergic stimulation of NCLX-null BAT, triggers the mitochondrial permeability transition pore (mPTP) opening, leading to a remarkable mitochondrial swelling and cell death. Treatment with mPTP inhibitors rescue mitochondrial function and thermogenesis in NCLX-null BAT, while calcium overload persists. Our findings identify a key pathway through which BA evade apoptosis during adrenergic stimulation of uncoupling. NCLX deletion transforms the adrenergic pathway responsible for thermogenesis activation into a death pathway.