Serum magnesium (MgS), levels were determined in 137 children age range 2 months to 16 years. Ultrafilterable magnesium (MgU) and intraerythrocytic magnesium (MgI) concentrations were determined in 37 of these children. MgS was 0.83 +/- 0.1 mmol/L (range 0.66-1.36 mmol/L), with no differences between sexes. Children under 2 years had higher MgS levels (0.92 +/- 0.13 mmol/L) than children over 2 years (0.81 +/- 0.08 mmol/L; p < .001). Mean MgU was 0.60 +/- 0.07 mmol/L (range 0.50-0.87 mmol/L), with no differences between sexes. Mean MgI in children was 2.58 +/- 0.33 mmol/L (range 2.06-3.6 mmol/L), with no differences between sexes. MgS correlated with MgU, age, theoretical growth rate, and serum calcium, phosphorus and alkaline phosphatase; MgU correlated with MgI, age, theoretical growth rate, and serum phosphorus. MgS concentration was higher in children under 2 years than in children over 2 years. In healthy children, MgS concentration correlated with MgU, and MgU correlated with MgI, but MgS and MgI showed no correlation.