In human pregnancies placental amino acid transport has been studied at the time of delivery and also by in utero fetal blood sampling (FBS). A significant reduction in amino acid fetal-maternal gradients and in umbilical veno-arterial differences has been demonstrated in intrauterine growth-restricted (IUGR) pregnancies. Fetal-maternal transfer rates have been further investigated in vivo by stable isotope methodologies. Following a maternal bolus infusion of [1-13C]-glycine and [1-13C]-leucine performed at fetal blood sampling, the transfer rate of the non-essential amino acid glycine is significantly lower than that for the essential amino acid leucine, suggesting that glycine can be newly synthesized in the feto-placental unit. Moreover, in growth-restricted pregnancies the fetal/maternal ratio of [1-13C]-leucine is significantly lower, and proportional to the degree of severity. In vitro studies have described a variety of transport systems for amino acids within the microvillous membrane (MVM) and the basal membrane (BM) of the placenta and significant differences have been reported in growth-restricted pregnancies for system A, system L, and taurine transporters. These changes are significantly associated to both biophysical and biochemical parameters of severity. Moreover, significant relationships can be found in arginine transport system and uterine oxygenation, suggesting a role in nitric oxide (NO) synthesis.