To test the hypothesis that restriction of sodium intake during the first 3 to 5 days of life will prevent the occurrence of hypernatremia and the need for administration of large fluid volumes, we prospectively and randomly assigned 17 babies (mean +/- SD: 850 +/- 120 gm; 27 +/- 1 weeks of gestation) to receive in blind fashion either daily maintenance sodium or salt restriction with physician-prescribed parenteral fluid intake. Maintenance-group infants received 3 to 4 mEq of sodium per kilogram per day; restricted infants received no sodium supplement other than with such treatments as transfusion. Sodium balance studies conducted for 5 days demonstrated that maintenance salt intake resulted in a daily sodium balance near zero, whereas sodium-restricted infants continued to excrete urinary sodium at a high rate, which promoted a more negative balance (average daily sodium balance -0.30 +/- 1.78 SD in maintenance group vs -3.71 +/- 1.47 mEq/kg per day in restriction group; p less than 0.001). Care givers tended to prescribe daily increases in parenteral fluids for the salt-supplemented infants, perhaps because serum sodium concentrations were elevated in these infants after the first day of the study (p less than 0.001). Hypernatremia developed in two sodium-supplemented infants (greater than 150 mEq/L), and hyponatremia developed in two sodium-restricted infants (less than 130 mEq/L); however, the restricted infants were more likely to have normal serum osmolality (p less than 0.05). Both groups of infants produced urine that was neither concentrated nor dilute, with a high fractional excretion of sodium; renal failure was not observed. The mortality rate was not affected, but the incidence of bronchopulmonary dysplasia was significantly less in the sodium-restricted babies (p less than 0.02). We conclude that in tiny premature infants, a fluid regimen that restricts sodium may simplify parenteral fluid therapy targeted to prevent hypernatremia and excessive administration of parenteral fluids.