Sodium balance was studied in 17 consecutively admitted neonates weighing less than 1,200 gm at birth. Infants whose gestation was less than or equal to 30 weeks were sicker and were in markedly negative sodium balance on day 3 (-9.25 mEq/kg day), despite a high sodium intake (7.22 mEq/kg/day). This negative balance was the result of a high fractional sodium excretion and resulted in hyponatremia in six (50%) of the patients. By day 8 these immature infants were in positive sodium balance, although fractional sodium excretion and daily sodium requirements remained high. More mature infants (greater than 30 weeks gestational age) were in positive sodium balance on both days 3 and 8. Creatinine clearance did not differ significantly between groups on either day 3 or 8 but increased within each group during the study period. These data suggest that the daily sodium requirement of immature sick infants may be much higher than was previously suggested.