The urinary activity of the angiotensin-converting enzyme (UACE) is not yet completely documented in human neonates. We measured the UACE in 36 premature neonates on the 1st day and in the 1st, 2nd, 3rd, and 4th weeks of life, in 22 full-term neonates between the 1st and 2nd days, and in 30 nursing and preschool children between 1 month and 6 years of age. The urinary excretion of sodium (UNa/UCr) and the potassium/sodium index (UK/UNa) were analyzed in the neonates. UACE was greater in premature than in full-term neonates and greater in both than in older children (p<0.001). In the premature neonates, UACE peaked at the 2nd week, the UNa/UCr index decreased, and the UK/UNa index increased between the 1st day and the 2nd week (p<0.001). The UNa/UCr index on the 1st day and in the 1st and 2nd weeks was greater in premature than in full-term neonates (p<0.001). There was no significant correlation between the UACE and the UNa/UCr index. In conclusion, the UACE profile was shown to be age dependent and related to the postnatal renal development. The increase in UACE activity may reflect the high activity of the neonatal intrarenal renin-angiotensin system (RAS).