We studied 16 women, at 32 weeks' or more gestation who required magnesium sulfate (MgSO4) therapy for preterm labor or preeclampsia. A 60-minute Doppler fetal heart rate (FHR) tracing, analyzed by the Oxford Sonicaid System 8000, was obtained for 1 hour before and 2 hours after each patient received intravenous MgSO4 therapy. Maternal serum Mg2+ levels were obtained at the second monitoring session. Matched paired measures of FHR parameters were compared with the Student's t test. After MgSO4 administration, we noted significant falls in long-term variability, short-term variability, and total acceleration (more than 10 beats/min) counts. Reduced short-term and overall variability occurred in all cases with maternal serum Mg2+ levels more than 4.6 mg/dL. Therapeutic maternal serum Mg2+ levels are linked with decreases in long-term and short-term FHR variability and acceleration counts. These findings should be considered when evaluating resting FHR baseline of patients thus treated.