The non-distressing [15N]methacetin liver function test was modified and applied to newborn healthy infants in order to measure both the total [15N]methacetin metabolites excreted in the urine (total elimination capacity) and the proportion of glucuronated metabolite. By studying pre- and full-term normotrophic neonates 3-168 days old, the age-dependent maturation of the two developing liver function processes can be compared on the basis of either postnatal or postmenstrual age. When solely considering postnatal age, no significant differences between the pre- and full-term infants were observed in the development of the total elimination capacity. However, when postmenstrual age was considered, it became apparent that this development starts earlier in pre-term infants and continues at the same rate as their full-term counterparts, up to the postmenstrual age of approximately 280 days. This increase subsequently diminishes in the pre-terms. In the same study group, the proportion of glucuronidation, another indicator of the hepatic detoxification system, appears to develop at a lower rate in pre-term than in full-term infants. When postmenstrual age is taken into consideration, glucuronidation development is also observed to begin earlier in pre-term infants and the slower maturation is more pronounced. Although these results are not generally applicable, they contribute to a better interpretation of the [15N]methacetin liver function test--for instance when estimating effects due to environmental exposure or accurately calculating age-related drug dosage for neonates.