Preeclampsia is frequently treated with the intravenous infusion of magnesium sulphate, which is well known to control seizures in the eclamptic patient. The effects of this drug on the cerebral vasculature, however, are not well defined. Doppler ultrasound allows the evaluation of changes in blood flow velocity in vessels previously inaccessible to researchers. This technology has recently been applied to the study of the retinal effects of parenteral magnesium sulphate. The efficacy of this agent as a small vessel cerebral vasodilator has been demonstrated, both in the middle cerebral artery and in the orbital vessels. This new technique may shed light on the pathophysiology of preeclampsia and on the mechanism of action of magnesium sulphate in this disease.