We find that the adrenergic agonist isoproterenol increases intracellular Ca²⁺ concentration ([Ca²⁺]_i) in cultured rat brown adipocytes. At the concentration used (10 µM), isoproterenol-induced Ca²⁺ responses were sensitive to block by either ₁- or -adrenergic antagonists, suggesting an interaction between these receptor subtypes. Despite reliance on -adrenoceptor activation, the Ca²⁺ response was not due solely to increases in cAMP because, administered alone, the selective ₃-adrenergic agonist BRL-37344 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 inositol 1,4,5-trisphosphate (IP₃)-sensitive Ca²⁺ stores, endoplasmic reticulum store depletion by thapsigargin blocked isoproterenol-induced Ca²⁺ increases, but removal of external Ca²⁺ did not. These results argue that increases in cAMP sensitize the IP₃-mediated Ca²⁺ release system in brown adipocytes.