Serial magnesium determinations by atomic absorption spectrophotometry were performed in the first five days of age on the serum of 91 infants with birth weights less than 2.0 kg (4.4 lb). There was a gradual rise in Mg levels with postnatal age, in both preterm infants with birth weights appropriate for gestational age and infants who were small for gestational age. Decreased Mg levels were observed in infants who were small for gestational age, and in infants born of toxemic mothers and primiparous mothers. There was an inverse relation between Mg levels and gestational age. There was no significant correlation between Mg levels and serum calcium, phosphorus, or protein levels. The mean of all determinations was 2.26 mg/100 ml ± SD 0.34. These figures are similar to adult values. Three infants with serum Mg values below 2 SD from the mean exhibited signs of increased neuromuscular irritability. The low Mg levels reverted to normal without treatment.