It is suitable to examine the utilization of carbohydrates and fats using stable isotope-labelled substrates in neonates because of their non-radioactivity. Administering medium-chain triglycerides (MCT) and oligosaccharides is of use in enteral nutrition for a patient with a limited water intake such as a neonate. In this study, the oxidation of MCT and maltose administered orally as an energy supplement in neonates has been examined using a stable isotope-labelled breath test. Five normal term neonates and five growing preterm infants were given [13C]-trioctanoin orally and three growing preterm infants were given [13C]-glucose and [13C]-maltose orally. The [13C] enrichment in carbon dioxide was analyzed by isotope ratio mass spectrometry, and oxidation rates over 6 hr and 12 hr respectively, were calculated. The oxidation rates for [13C]-octanoin after 6 hr were 46.2 +/- 3.6% in preterm infants and 53.5 +/- 13.8% in normal neonates, respectively (no significant difference), and 58.4 +/- 9.4% and 52.8 +/- 6.0% for [13C]-glucose and [13C]-maltose, respectively (not significant). The results demonstrate that orally administered MCT and maltose are oxidized sufficiently in preterm infants.