Magnesium sulfate (MgSO4) induces relaxation of pulmonary vessels but the mechanism of action is still unknown. To explore possible mechanisms of action of MgSO4 in pulmonary vessels, we studied isolated third and fourth generation pulmonary arteries (PA) from newborn rabbits (3-4 weeks old). PA rings were suspended in organ chambers filled with modified Krebs solution (95% O2, 5% CO2, 37°C) and their isometric tension recorded. Contraction was induced with 3×10-9M endothelin-1 and relaxation with increasing concentrations of MgSO4 (1.2 to 8 × 10-3 M). The effect of indomethacin, N-nitro-L-arginine and thapsigargin and ryanodine, two modulators of intracellular calcium stores, on MgSO4 relaxation was tested. MgSO4 (2 to 8 × 10-3 M) induced progressive relaxation of PA (4±2 to 48±3%) in a dose dependent manner. Maximal relaxation induced by MgSO4 was not affected by indomethacin or N-nitro-L-arginine, suggesting that the relaxant effect of magnesium on pulmonary arteries is not mediated through the cyclooxygenase pathway nor the endothelium. Magnesium-induced relaxation was also not affected by thapsigargin and ryanodine, which inhibit the release of calcium from intracellular stores, suggesting that increased magnesium does not induce relaxation through modulation of intracellular calcium.