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Infants born early with very low birth weights frequently have high amounts of sodium in their urine and an increased loss of the fluid ordinarily found between the cells, called extracellular fluid. The amount of extracellular fluid is controlled by the balance between the amount of sodium taken in and that excreted by the kidneys. Although this condition is common in preterm infants, it is difficult to assess whether this is the result of disease or a natural response in infants born too early. To determine whether infants should receive small amounts of sodium to help reduce the extracellular fluid or increased amounts of sodium to replace sodium lost by the kidneys, 20 preterm infants were evaluated. The extracellular fluid volume (ECV) and the balance of sodium (intake and output) were measured for 10 days in two groups of infants, one receiving small and the other large amounts of sodium. During the first five days following birth, the ECV, which was normally high at birth in preterm infants, was reduced in both groups, creating a net loss of sodium. Five days later, the sodium output decreased in both groups, making the sodium balance positive. The ECV increased proportionately as the infant grew. There was a lower amount of sodium found in the blood of those infants taking in low sodium. It is suggested that low-birth-weight infants can regulate the excretion of sodium, but cannot control the concentration of sodium found in the blood. The role of hormones which regulate sodium balance cannot be overlooked, although the amounts of aldosterone and vasopressin, both sodium-controlling hormones, were not significantly different in the both groups. The adjustments made by the preterm infant to regulate the amount of ECV constitute a normal physiological response to life outside of the uterus.