Magnesium is a natural calcium channel blocker inhibiting vasoconstriction in numerous vascular beds. Magnesium sulphate given prior to birth to pre-eclamptic mothers and mothers in preterm labour has in retrospect been found to be associated with a decreased incidence of both intraventricular haemorrhage and cerebral palsy. Little is known about the effect of normal variations of serum magnesium in the very preterm baby, where morbidity is closely related to rapid vascular changes. We have analysed the absolute levels and normal variations of magnesium concentration in cord blood and during the first 3 weeks after birth for 69 infants born before 32 gestational weeks of age. The results show an inverse relation between serum magnesium at birth and gestational age. Higher levels of serum magnesium at birth within normal variations were associated with a delayed closure of the ductus arteriosus, and mild but not severe peri- and intraventricular haemorrhage.