Small for gestational age (SGA) infants are known to develop relatively mild transient hyperbilirubinaemia, especially in comparison with premature infants. This may be interpreted as an index of accelerated maturation of particular vital functions. In the present study 12 SGA infants, 12 appropriate for gestational age (AGA) infants, and 12 premature infants had 24 h urine collections under standardized conditions on the third day of life. Urinary excretion of D-glucaric acid and creatinine, which can be increased by drugs (e.g. phenobarbital), were estimated. Haematocrit and bilirubin were determined at the age of 72h. In the 3 groups, the SGA infants had the highest mean haematocrit (SGA = 0.58, AGA = 0.5, Prem = 0.561/1) and the lowest mean bilirubin (SGA = 6.3, AGA = 8.6, Prem = 11.1 mg/dl). Despite a comparatively small urine volume (SGA = 19.6, AGA = 28.3, Prem = 37.3 ml/kg), excretion of D-glucaric acid (SGA = 0.18, AGA = 0.11, Prem = 0.06 mumol/kg) and creatinine (SGA = 101.6, AGA = 79.6, Prem = 80.2 mumol/kg) was significantly higher in the SGA infants. While the results do not provide conclusive evidence, the increased excretion of glucaric acid and creatinine associated with mild transient hyperbilirubinaemia can be considered an indicator of metabolic differences in SGA infants.