BACKGROUND AND PURPOSE: Magnesium sulfate is used extensively for prevention of eclamptic seizures. Empirical and clinical evidence supports the effectiveness of magnesium sulfate; however, questions remain as to its safety and mechanism. This review summarizes current evidence supporting the possible mechanisms of action and several controversies for magnesium sulfate treatment. SUMMARY OF REVIEW: Several mechanisms are presented, including the effects of magnesium sulfate on peripheral and cerebral vasodilation, blood-brain barrier protection, and as an anticonvulsant. CONCLUSIONS: Though the specific mechanisms of action remain unclear, the effect of magnesium sulfate in the prevention of eclampsia is likely multi-factorial. Magnesium sulfate may act as a vasodilator, with actions in the peripheral vasculature or the cerebrovasculature, to decrease peripheral vascular resistance or relieve vasoconstriction. Additionally, magnesium sulfate may also protect the blood-brain barrier and limit cerebral edema formation, or it may act through a central anticonvulsant action.