Magnesium is an accepted drug in the management of toxemia of pregnancy, and is employed by many physicians as the anticonvulsant agent of choice in the therapy of this commonly encountered gestational disease. Many modes of administration and variable dosage schedules have been evolved over the past half century, ranging from small quantities given orally to relatively large amounts introduced by the parenteral route.In a recent study conducted at the University of Virginia Hospital,5 it was suggested that prompt, rapidly attained, and effective magnesium blood levels (7 to 8 mEq. per liter) could best be effected by introducing this agent intravenously. Therefore, during the last 30 months, pre-eclamptic and eclamptic patients admitted to the Obstetric and Gynecologic Service of the University Hospital have received magnesium intravenously in high dosages.Clinical observations carried out in our labor and delivery suites have, however, given the impression that the intravenous administration of magnesium sulfate, in sufficient concentration, results in a prolongation of labor. Inasmuch as the commonly utilized sedative drugs morphine, meperidine, and phenobarbital used in the therapy of toxemia have been seemingly exonerated of a direct uterine depressant activity,14 and since the only departure from the customary therapeutic regimen during the past 30 months from that employed previously was the elevation of the serum magnesium level, it appeared probable that, if labor were in truth prolonged in these patients, the magnesium ion was depressant to the human myometrium during labor. It was in an attempt to clarify this point that the present study was undertaken.