Background: The normal development of the uteroplacental circulation in pregnancy depends on angiogenic and vasodilatory factors such as vascular endothelial growth factor (VEGF). Reduced uterine artery blood flow (UABF) is a common cause of fetal growth restriction; abnormalities in angiogenic factors are implicated. Previously we showed that adenovirus (Ad)-mediated VEGF-A 165 expression in the pregnant sheep uterine artery (UtA) increased nitric oxide synthase (NOS) expression, altered vascular reactivity and increased UABF. VEGF-D is a VEGF family member that promotes angiogenesis and vasodilatation but, in contrast to VEGF-A, does not increase vascular permeability. Here we examined the effect of Ad.VEGF-D ΔNΔC vector encoding a fully processed form of VEGF-D, on the uteroplacental circulation. Methods: UtA transit-time flow probes and carotid artery catheters were implanted in mid-gestation pregnant sheep (n = 5) to measure baseline UABF and maternal haemodynamics respectively. 7-14 days later, after injection of Ad.VEGF-D ΔNΔC vector (5×10 11 particles) into one UtA and an Ad vector encoding β-galactosidase (Ad.LacZ) contralaterally, UABF was measured daily until scheduled post-mortem examination at term. UtAs were assessed for vascular reactivity, NOS expression and endothelial cell proliferation; NOS expression was studied in ex vivo transduced UtA endothelial cells (UAECs). Results: At 4 weeks post-injection, Ad.VEGF-D ΔNΔC treated UtAs showed significantly lesser vasoconstriction (E max 144.0 v/s 184.2, p = 0.002). There was a tendency to higher UABF in Ad.VEGF-D ΔNΔC compared to Ad.LacZ transduced UtAs (50.58% v/ s 26.94%, p = 0.152). There was no significant effect on maternal haemodynamics. An increased number of proliferating endothelial cells and adventitial blood vessels were observed in immunohistochemistry. Ad.VEGF-D ΔNΔC expression in cultured UAECs upregulated eNOS and iNOS expression. Conclusions: Local over-expression of VEGF-D ΔNΔC in the UtAs of pregnant mid-gestation sheep reduced vasoconstriction, promoted endothelial cell proliferation and showed a trend towards increased UABF. Studies in cultured UAECs indicate that VEGF-D ΔNΔC may act in part through upregulation of eNOS and iNOS.