Magnesium, the second most abundant intracellular cation, is essential for life. The consequences of deficiency are severest in the smallest and youngest members of each species and may include sudden unexpected death. Magnesium deficiency, usually diagnosed by hypomagnesemia, may be congenital, as in premature infants, infants of magnesium-deficient mothers and infants with intrauterine growth retardation. It may be acquired or caused by low magnesium intake, the use of magnesium-wasting drugs, illness provoking gastrointestinal or renal losses of the mineral, or high metabolic demands imposed by catch-up growth or postsurgical healing. Finally, the deficiency may be conditioned, caused by excessive dietary calcium, phosphorus or protein in relation to dietary magnesium, especially during a period of rapid growth or tissue repair. Magnesium therapy is safe when a low dosage is given with monitoring of plasma or serum magnesium levels, with occasional checking of calcium and potassium levels. A parenteral dose of 0.1 ml/kg/day of 50% magnesium sulfate USP (approx. 0.2 mmol/kg/day or 0.4 mEq/kg/day) may be given for 5 dose days. An oral dose of 1.0 ml of 10% magnesium chloride solution providing 0.5 mmol/kg/day magnesium or 1.0 ml/kg/day of 10% magnesium chloride USP (0.5 mmol/kg/day) or magnesium magonate (Magonate) 1.0 ml/kg/day (0.45 mmol/kg/day) may be given for extended periods; higher doses may be required for malabsorption syndromes. Hypermagnesemia, which usually results from magnesium overdosage or inadequate renal function, is a potential threat to neonates born to magnesium-treated eclamptic mothers. Most show marked improvement after 36 h of conservative management that includes calcium salts and intravenous infusions of glucose and saline, but obtunded neonates may require dialysis.