We undertook this in vitro study to quantify the effect that addition of magnesium (Mg) sulfate to neonatal serum, as compared with adult serum, might have on ionized Mg (Mg (2+)) concentration. We used one cord blood sample and one adult blood sample that we made hypermagnesemic by adding various amounts of magnesium sulfate to study five levels of serum Mg. Each sample was then studied at one of three levels of pH, from extreme alkalosis to extreme acidosis. We measured the changes in serum Mg (2+) and serum ionized calcium (Ca (2+)) in reaction to these changes in pH and Mg. At each pH level, there was an exponential increase in the serum Mg (2+) (and no significant change in serum Ca (2+)) with increasing serum Mg. Multiple regression analysis using Mg (2+) as the dependent variable and baseline Ca (2+), phosphorous, albumin, and blood type (adult versus cord blood) as well as blood pH and serum Mg as independent variables indicated that serum Mg and pH were the only variables significantly influencing serum Mg (2+). Within the range of serum Mg considered, the addition of magnesium sulfate in vitro causes an exponential increase in Mg (2+) and no significant change in serum Ca (2+).