Mg2⁢ is the fourth most abundant cation in cellular organisms. Although the biological chemistry and the physiological roles of the magnesium ion were well known, the regulation of intracellular Mg2⁢ in mammalian cells is not fully understood. More recently, however, the mechanism of Mg2⁢ mobilization by hormonal stimulation has been investigated in hearts and in myocytes. In this work we have investigated the regulation mechanism responsible for the Mg2⁢ mobilization induced by α1-adrenoceptor stimulation in perfused guinea pig hearts or isolated myocytes. The Mg2⁢ content of the perfusate or the supernatant was measured by atomic absorbance spectrophotometry. The elimination of Mg2⁢ in the medium increased the force of contraction of right ventricular papillary muscles. Phenylephrine also enhanced the force of contraction in the presence of Mg2⁢-free medium. α1-Agonists such as phenylephrine were found to induce Mg2⁢ efflux in both perfused hearts or myocytes. This was blocked by prazosin, a α1-adrenoceptor antagonist. Mg2⁢ efflux by phenylephrine was amplified by Na⁢ channel blockers, an increase in extracellular Ca2⁢ or a decrease in extracellular Na⁢. By contrast, the Mg2⁢ influx was induced by verapamil, nifedipine, ryanodine, lidocaine or tetrodotoxin in perfused hearts, but not in myocytes. W7, a Ca2⁢/calmodulin antagonist, completely blocked the pheylephrine-, A23187-, veratridine-, Ca2⁢-induced Mg2⁢ efflux in perfused hearts or isolated myocytes. In addition, Mg2⁢ efflux was induced by W7 in myocytes but not in perfused heart. In conclusion, An increase in Mg2⁢ efflux by α1-adrenoceptor stimulation in hearts can be through IP3 and Ca2⁢-calmodulin dependent mechanism.