Urinary N-acetyl-beta-glucosaminidase (NAG) excretion was measured in 14 healthy, preterm, male neonates with gestational ages between 32 and 35 weeks. Daily NAG excretion increased significantly during the first four weeks of life. No correlation was observed between urinary NAG:creatinine ratio and postnatal age regardless of whether measurements were taken from the whole 24 hour urine collection or from an isolated urine spot sample at the same time on each day. When the preterm infants were compared with a group of 20 healthy, full term, male infants at a postnatal age of 7 days the NAG:creatinine ratio was significantly higher in the preterm group, the measurements having been taken from single urine spot samples. We suggest that this variable be used in the evaluation of renal tubular integrity during the neonatal period.