The purpose of this study was to design the shortest suitable magnesium load test for infants up to six months of age. A 56-hour test was used to study cation and creatinine excretion before and after an intramuscular load of 0.49 mEq of magnesium/kg of body weight. No diurnal excretion pattern for magnesium was found, and most of the magnesium rejected by the kidney was excreted by neonates within 32 hours, and by infants from one to six months of age within 24 hours. The postload urinary magnesium value usually stabilized at a slightly higher level than the preload level. The mean plasma mag nesium level increased about 0.65 mEq/liter one hour after the load, and at the end of the test was 0.25 mEq/liter higher than the initial value; abnormally low plasma values were normalized. Renal excretion of creatinine and potassium was not affected by the magnesium load, but the 24-hour postload excretion was higher than the 24-hour preload excretion of calcium for all patients, and of sodium for some full-term neo nates. Because of slow excretion of magnesium and concomitant calcium loss, special precau tions are required for testing and repletion of young infants to avoid magnesium overload and calcium depletion. A suitable test would have an eight-hour preload period, a load of 0.49 mEq of magnesium (0.12 ml of 50 per cent MgSO4.7H2O) per kg of body weight, and a postload collection of 32 hours for neonates and 24 hours for infants from one to six months of age.
