Adenosine is a potent activator of human and murine brown adipocytes, but nothing is known how adenosine levels are regulated in thermogenic fat. Here, we will study the role of the highly expressed equilibrative nucleoside transporter 1 (ENT1) for thermogenic adipocytes and for whole-body metabolism. Our preliminary data indicate that ENT1 plays a major role for extracellular adenosine levels in brown adipocytes. Moreover, we will study how human ENT1 mutations affect extracellular adenosine levels. Additionally, we will scrutinize the contribution of non-adipocytes (e.g. endothelial cells, macrophages) present in thermogenic fat to overall adenosine content.