Objective:To determine the relation between magnesium sulfate therapy and fetal heart rate (FHR) variability.Methods:Twelve women diagnosed with preeclampsia who were to receive magnesium sulfate for seizure prophylaxis were recruited. Exclusion criteria included delivery anticipated within 2 hours, gestational age less than 28 weeks, abnormal fetal testing, fetal growth retardation, and use of illicit drugs or medications. Six grams of magnesium sulfate in 100 mL 0.9% saline was administered intravenously over 20 minutes, followed by a continuous infusion of 2 g/hour. A cardiotocogram analysis computer system was used to analyze various elements of the FHR pattern before, during, immediately after, and 60 minutes following magnesium sulfate loading. For each time interval, the basal heart rate, number of accelerations and decelerations, number of minutes of high and low variability, mean minute range variation, and the short‐term (3.75‐second interval) variability were measured.Results:There was a statistically significant decrease in short‐term variability 60 minutes after initiation of therapy (6.7 ± 2.0 versus 9.8 ± 3.3 milliseconds;P= .003). Long‐and medium‐term variability did not change significantly after magnesium sulfate was administered.Conclusion:Although magnesium sulfate therapy was associated with an objectively measured, statistically significant decrease in short‐term variability, the decrease was not clinically significant; furthermore, it was not associated with a decrease in long‐term variability or in the number of accelerations measured.(Obstet Gynecol 1994;83:967‐70)