We find that the adrenergic agonist isoproterenol (ISO) increases intracellular calcium ([Ca²⁺]_i) in cultured rat brown adipocytes. At the concentration used (10 µM), isoproterenol-induced calcium responses were sensitive to block by either ₁- or ß-adrenergic antagonists, suggesting an interaction between these receptor subtypes. Despite reliance on ß-adrenoceptor activation, the calcium response was not due solely to increases in cAMP because, administered alone, the selective ß₃-adrenergic agonist BRL37344 or forskolin did not increase [Ca²⁺]_i. However, increased cAMP elicited vigorous [Ca²⁺]_i increases in the presence of barely-active concentrations of the -adrenergic agonist phenylephrine or the P2Y receptor agonist UTP. Consistent with isoproterenol recruiting only IP₃-sensitive calcium stores, ER store depletion by thapsigargin blocked isoproterenol-induced calcium increases, but removal of external calcium did not. These results argue that increases in cAMP sensitize the IP₃-mediated Ca²⁺ release system in brown adipocytes. These results argue that increases in cAMP sensitize the IP₃-mediated Ca²⁺ release system in brown adipocytes.