L-Tryptophan (L-Trp) is a precursor for serotonin (5-HT) and nicotinamide adenine dinucleotide (NAD) synthesis. Both 5-HT and NAD may impact energy metabolism during gestation given that recent studies have demonstrated that increased 5-HT production is crucial for increasing maternal insulin secretion, and that sirtuin, an NAD(+)-dependent protein deacetylase, regulates endocrine signaling. Infants born with intrauterine growth restriction (IUGR) are at a higher risk of metabolic disease once they reach adulthood. IUGR is associated with altered maternal-fetal amino acid transfer. Whether IUGR affects L-Trp metabolism in mother and fetus has not been fully elucidated. Recently, we developed an analytical method using stable isotopelabeled L-Trp to explore the metabolism of L-Trp and its main metabolites, L-kynurenine (L-Kyn), 5-HT and quinolinic acid (QA). In this study, dams submitted to dietary protein restriction throughout gestation received intravenous infusions of stable isotope-labeled N-15(2)-L-Trp to determine whether L-Trp metabolism is affected by IUGR. Samples were obtained from maternal, fetal and umbilical vein plasma, as well as the amniotic fluid (AF), placenta and liver of the mother and the fetus after isotope infusion. We observed evidence for active L-Trp transfer from mother to fetus, as well as de novo synthesis of 5-HT in the fetus. Plasma 5-HT was decreased in undernourished mothers. In IUGR fetuses, maternal-fetal L-Trp transfer remained unaffected, but conversion to QA was impaired, implying that NAD production also decreased. Whether such alterations in tryptophan metabolism during gestation have adverse consequences and contribute to the increased risk of metabolic disease in IUGR remains to be explored.