['Although of pharmacokinetic and -dynamic relevance, data on ontogeny of UDP-glucuronosyltransferase (UGT) activity in neonates are scant. We therefore wanted to assess the impact of both postnatal and postmenstrual age (PNA/PMA) on the interindividual variability of glucuronidation to overall tramadol urinary elimination in neonates.', 'O-demethyl tramadol (M1) and M1-glucuronide (M1G) were determined in 24 hour urine collections during continuous intravenous tramadol administration in neonates. Glucuronidation fraction (%) was calculated by the ratio of M1G to the sum of M1G and M1 free (M1total). Fractions (%) in early (<day 8) or late neonatal life (day 8-28) were compared (Mann-Whitney U) and forward multiple regression was applied to assess the impact of various covariates.', 'Urine collections were available in 59 neonates with a PNA of 6 (1-28) days and a PMA of 38 (SD 4) weeks. Mean M1G/M1total was 27 (SD 15) % and was significantly lower in early compared to late neonatal life (22 versus 32%, p=0.0001). In a forward multiple regression model, both PMA and early versus late neonatal life remained independent variables to explain the interindividual variability in M1G/M1total.'] Do both postnatal and postmenstrual age contribute to the interindividual variability in tramadol glucuronidation in neonates?