Magnesium (Mg) catalyzes or activates more than 300 enzymes in the body, and it plays a pivotal role in the metabolism of carbohydrates, fats and proteins. Until recently Mg has only been measured as a total substance concentration; however, it is the free ionized form of magnesium (iMg2+) that is physiologically active. In 1992, a novel ion selective electrode (ISE) utilizing a neutral carrier-based membrane for assessment of iMg2+ in whole blood, plasma or serum was designed and it was used to measure the iMg2+ level in whole blood of 160 healthy Japanese children. There was a significant positive correlation between serum total magnesium (SMg) and iMg2+. Percent ionized magnesium (iMg2+/SMg) was 58.3 +/- 4.1% and increased with growth (age and bodyweight). The level of iMg2+ was 1.29 +/- 0.08 mg/dL (range, 1.09-1.51) ¿0.535 +/- 0.033 mmol/L (range, 0.452-0.627)¿ and was constant irrespective of growth. On the other hand, SMg, serum total calcium (SCa) and ionized calcium (iCa2+) decreased with growth. Thus only iMg2+ level was constant irrespective of growth. It is suspected that the buffer action of protein binding Mg may keep the iMg2+ level constant, because iMg2+ plays an important part in the activation of many enzymes.