The plasma creatinine concentration is elevated at birth and decreases concomitantly with the rapid increase in glomerular filtration rate that occurs in the first postnatal weeks. The velocity of these changes was assessed during the first 3 wk of life of 66 term and preterm infants. The plasma creatinine concentration, creatinine clearance, and sodium fractional excretion were measured serially at weekly intervals, starting 1–4 d after birth [mean = 1.5 ± 0.8 (SD) d]. Premature infants were separated into three groups according to their birth weight: group 1, 1001 to 1500 g; group 2, 1501 to 2000 g; and group 3, 2001 to 2500 g. Group 4 included 28 term infants (mean birth weight = 3165 ± 78 g). Mean gestational ages in the preterm groups were 31.3, 32.8, and 34.4 wk in groups 1, 2, and 3, respectively. The plasma creatinine concentration on d 1.5 was significantly higher in preterm (91 ± 4 μmol/L) compared with term infants (66 ± 3 μmol/L). The differences in plasma creatinine were still present during the second week of life, with values of 64 ± 5, 58 ± 7, 47 ± 8, and 40 ± 4 μmol/L in groups 1, 2, 3, and 4, respectively. The difference vanished by d 22–23. On d 1.5, creatinine clearance correlated positively with gestational age, amounting to 0.65 ± 0.14, 0.92 ± 0.19, 1.42 ± 0.31, and 3.36 ± 0.32 mL/min in groups 1, 2, 3, and 4, respectively. Creatinine clearance increased rapidly with postnatal age, the velocity of the maturation being less marked in the most premature infants. The fractional excretion of sodium was significantly higher in the most premature infants, with values of 2.0 ± 0.3, 2.2 ± 0.5, 1.1 ± 0.2, and 0.3 ± 0.1% in groups 1, 2, 3, and 4, respectively. The differences vanished by the third week of life. The negative correlation between plasma creatinine on d 1.5 and gestational age suggests that the neonate's creatinine plasma concentration does not simply reflect the mother's plasma concentration.