ATP can act in a paracrin/autocrine fashion on P2Y receptors on endothelial cells (ECs) to elicit release of nitric oxide and prostaglandins, which cause dilatation of the vascular smooth muscle (VSM). However, ATP can also act directly on P2X receptors on VSM to cause contraction (Burnstock, 1990) and in human umbilical artery VSM cells, ATP can induce [Ca2+]i oscillations that are generally considered to induce vasomotion (Meng et al., 2007). A hypoxic intrauterine environment is associated with poor fetal O2 and nutrient delivery and leads to disorders such as intrauterine growth retardation and pre-eclampsia. Further, such conditions are associated with increased vasomotion (Hempstock et al., 2003). Thus, the question arises as to whether hypoxia can induce release of ATP from the EC of umbilical vessels. In the present study, EC were freshly isolated from human umbilical arteries (HUAEC) and veins (HUVEC). (Luu et al., 2010). These were separately grown on culture inserts with pore size of 0.4 µm and pore density of 2.0 ± 0.2 x 106 / cm2. After rinsing with sterile PBS, they were placed in fresh culture medium and then exposed to 1% O2, 5% CO2, balanced with N2 at 37 °C for 30 minutes (hypoxia), or kept in normal culture conditions (normoxia). Triplicates of cells from three individual donors were used for each series of experiments. Samples of 50 or 100 µl of medium were taken from the apical and the basolateral side of each culture insert. These were then assayed for ATP by using the highly sensitive luciferin-luciferance reaction (CellTiter-Glo® Reagent from Promega). The strength of luminescence signal was recorded and ATP concentration was calculated from a standard curve generated for each series of experiments. Statistical analysis was carried out using Student’s t-test. There was no difference in ATP released in normoxia and hypoxia from HUAEC (Apical: 1.0% O2: 21% O2: 11.0 ± 3.33 nM; 14.6 ± 3.33 nM; P= 0.18. N=6, where N = number of cords). Basolateral: 21% O2: 0.37 ± 0.033 nM; 1.0% O2: 0.42 ± 0.075 nM. P= 0.20. N=6). However, hypoxia for 30 mins significantly increased ATP release from both the apical (21% O2: 0.435 ± 0.08 nM; 1.0% O2: 3.04 ± 0.91 nM. P= 0.01. N=3) and basolateral (21% O2: 0.04 ± 0.001 nM; 1.0% O2: 0.11 ± 0.01 nM. P< 0.001. N=3) side of HUVEC. These results indicate that acute hypoxia can induce ATP release from the basolateral and especially from the apical surface of HUVEC, but not from HUAEC. This leads us to propose that a fall in pO2 in the umbilical vein that carries blood to the fetus from the placenta may be a mechanism by which ATP can be released and act in a paracrine way to induce vasomotion.