Forty term pregnant women with singleton breech gestations admitted for external cephalic version underwent biophysical profile testing before any fluid infusion or medication. After magnesium sulfate had been infused for contraction prophylaxis, the maternal serum magnesium level was measured and a second biophysical profile was performed. The mean (± SD) serum magnesium was 5.1 ± 1.0mg/dL. The biophysical profile score decreased significantly, reflecting a decrease in fetal breathing activity. In patients with therapeutic maternal serum magnesium levels, loss of any component of the biophysical profile other than respiration cannot be attributed to the elevation in magnesium concentration.