Magnesium sulfate, a biologically potent compound, given sometimes in extraordinarily high doses, is among the most commonly used pharmaceuticals in American obstetric practice. Although most clinicians are in accord regarding its value for seizure prophylaxis in the setting of preeclampsia, such unanimity is not the case regarding its role in preterm labor. Credible scientific data indicate not only a lack of efficacy, but also toxicity to susceptible fetuses when magnesium sulfate is used in the high dosages found in tocolysis. In apparent contrast, three recent clinical trials, although individually inconclusive, provide data from which a very recent meta-analysis affirms a potential role for magnesium sulfate in prophylaxis against fetal neurologic injury. Comparing outcomes from these trials, with attention to dosage, relationships are revealed that unify observations previously regarded as conflicting: Magnesium sulfate indeed may have both neuroprotective and fetal toxic effects. The better, and safer, neuroprotection seems to occur at comparatively low antenatal doses (perhaps in a range between 4 g and 10.5 g), whereas increasing dosages exceed a “therapeutic window” whereby, as with most drugs, toxic sequelae begin to accrue.