Intrauterine growth restriction (IUGR) is a pregnancy disturbance related to reduced foetal growth and hypoxia. IUGR is associated with endothelial dysfunction resulting in reduced synthesis of nitric oxide (NO) (Casanello & Sobrevia, 2002). In human umbilical vein endothelium (HUVEC) NO derives from conversion of L-arginine to L-citrulline via endothelial NO synthase (eNOS) and plays a crucial role in the pathogenesis of IUGR (Diket et al. 1994). L-Arginine transport via system y+/CATs (Cationic Amino acid Transporters) is inhibited by hypoxia in pulmonary artery endothelium (Zharikov & Block, 2000). We studied the effect of hypoxia on the activity and expression of the L-arginine/NO pathway in HUVEC from foetuses with IUGR. HUVEC from normal or IUGR pregnancies (Ethics committee approval and informed patient consent were obtained) were cultured in M-199 (20% bovine sera), and exposed (0-24 h) to normoxia (5% O2, ~35 mmHg PO2 in the umbilical vein in vivo, Mildenberger et al. 2003) or hypoxia (2% O2) obtained in a hypoxia chamber provided with an O2 sensor and hypoxic gas mixture (5% CO2, 95% N2). L-arginine transport (L-[3H]arginine, 5-1000 μM, 2 μCi/ml, 37°C, 1 min) was determined. hCAT-1, hCAT-2B and eNOS mRNA were quantified by real-time PCR. eNOS activity was determined by the L-[3H]citrulline assay (L-[3H]arginine, 4μCi/ml, 30 min), and total and phosphorylated eNOS (Ser1177) protein were detected by Western blot. Maximal transport velocity (Vmax) for L-arginine transport (5.0±0.3 pmol/μg protein/min, means±S.E.M., n=10-12) was reduced (P<0.05, unpaired t test) by IUGR (3.6±0.2 pmol/μg protein/min) and hypoxia (2.8±0.2 pmol/μg protein/min). However, hypoxia did not alter IUGR-reduced transport (3.8±0.2 pmol/μ protein/min). hCAT-1, hCAT-2B and eNOS mRNA were reduced (P<0.05, n=6-12) in IUGR (69±6, 70±7 and 73±5%, respectively) and hypoxia (50±4, 44±5 and 82±7%, respectively), but IUGR-induced reduction in hCATs and eNOS mRNA was not further altered by hypoxia (52±3, 40±3 and 86±5%, respectively). Total eNOS protein levels were increased by IUGR (1.7-fold) and hypoxia (1.3-fold), while phosphorylated eNOS at Ser1177 was reduced (P0.05, n=4) effect of hypoxia on IUGR-induced reduction of NO synthesis (69±6%). In summary, IUGR and hypoxia reduced L-arginine/NO pathway activity may result from lower mRNA expression of hCAT-1 and hCAT-2B transporters and eNOS as well as eNOS activity in HUVEC.