Introduction Pulmonary hypertension (PH) is a common complication of many chronic lung diseases which significantly reduces life expectancy. Previous work identified an important role for Gremlin1, a Bone Morphogenetic Protein antagonist, in the development of PH (Costello et al., 2008). Furthermore, ubiquitous haplodeficency of Gremlin1 in mice resulted in augmented BMP signalling and significantly reduced pulmonary vascular resistance (PVR) after 3 weeks of hypoxia exposure (Cahill et al., 2012). Gremlin within the lung is produced by endothelial cells (EC), type II pneumocytes and macrophages. Microvascular EC play an important role in the vascular changes associated with hypoxia. The specific role of endothelial dervied gremlin1 in the development of pulmonary hypertension is unknown nor is it known if the different sources within the lung are functionally redundant. To define the role of endothelial-derived gremlin in PH, an inducible endothelial gremlin deleted model was used. Methods and Results A novel inducible endothelial gremlin knock-out mouse model was generated by crossing mice where grem1 was flanked by loxP sequences (Gazzerro et al., 2007) with mice expressing the Cre recombinase under the control of the vascular endothelial cadherin promoter. Adult mice (age 6 weeks, n=10 per group) received chow with added Tamoxifen or a normal chow diet for 4 weeks, followed by a 2-week “wash-out” period on a normal chow diet. Groups of mice were then exposed either to 3 weeks normoxia or hypoxia (FIO2=0.10). Gremlin deletion was confirmed by the detection of the appropriate truncated amplicon. Mice were anaesthetised with urethane (1500mg.kg-1), anti-coagulated using heparin (1000 I.U.kg-1) and euthanased by ex-sanguination. Similar elevation of haematocrit was observed in both hypoxic groups (tamoxifen or normal diet) confirming a normal haemotopoeitic response to hypoxia. PVR was measured using the isolated ventilated lung preparation, perfused at a constant flow rate. Hypoxic exposure significantly increased haematocrit levels and the PVR of mice fed either a tamoxifen diet or normal diet (P<0.01). Mice with a reduced endothelial gremlin expression, fed tamoxifen, had an increased PVR compared to normal diet fed mice (P=0.023) following both normoxic and hypoxic exposure. Conclusion In conclusion, endothelial gremlin deletion increased pulmonary vascular resistance under both normoxic and hypoxic conditions suggesting that it plays an important role in normal pulmonary vascular homeostasis.