Inulin clearance (Cin) was measured in 20 infants of 27--43 weeks of gestation during the first 24 hr of their extrauterine life. Cin ranged from 0.7--4.7 ml/min and correlated with gestational age (P less than 0.05). In 18 infants of similar gestational ages studied after 2--3 days of extrauterine adaptation, the Cin ranged from 1.1--17.9 ml/min and also correlated with gestational age (P less than 0.01). By day 2--3, the infants of near term gestational age achieved a greater increase in Cin than did the markedly preterm infants, as reflected by a significantly different slope of the regression line for Cin and gestational age for the infants studied at 1 vs. 2--3 days of age (P less than 0.001).