Changes in accumulated fluid and sodium balance during intravenous maintenance fluid therapy has been studied in 38 newborn infants with different clinical disorders and gestational ages 28-42 weeks. The results from the infants born before 36 weeks of gestation (preterm) have been compared with the result from infants born after 36 weeks. Three different saline infusions 10, 20 and 40 mEq Na1/2/1000 ml 5.5% glucose have been given. The infusion rate has in preterm neonates been 3.3 ml/kg and hour and in the more full-term neonates been 3.6 ml/kg and hour. The study lasted for 5-8 hours. Urine was collected by spontaneous voidings in plastic bags. The balances were calculated as the difference between the amount given intravenously, and the amount excreted in the urine. In the more full-term neonates Na1/2 balance became increasingly negative with the 10 mEq solution, just balanced with the 20 mEq solution and increasingly positive with the 40 mEq solution. A different response was found in the preterm neonates. The natriuresis was higher and the sodium balances were increasingly negative with both the 10 and 20 mEq solutions. With the 40 mEq solution the negative balance tended to level off. The fluid balances were fairly well maintained in all infants regardless of the sodium concentration in the infusate.