Abstract-The stimulation of ␤-adrenoceptors in cardiac cells results in a rapid loss of cellular Mg 2ϩ . Because insulin physiologically counteracts several of the cellular effects mediated by the activation of ␤-adrenoceptors and the elevation of cytosolic cAMP levels, we investigated whether insulin administration could prevent Mg 2ϩ mobilization from rat hearts and ventricular myocytes. Rat hearts were perfused in a retrograde Langendorff system, and the changes in extracellular Mg 2ϩ were measured by atomic absorbance spectrophotometry. Pretreatment of the hearts with 6 nmol/L insulin completely prevented the Mg 2ϩ extrusion induced by the ␤-adrenergic agonist isoproterenol.