Chronic hypoxia causes pulmonary hypertension associated with structural alterations in pulmonary vessels and sustained vasoconstriction. The transcriptional mechanisms responsible for these distinctive changes are unclear. We have previously reported that CREB1 is activated in the lung in response to alveolar hypoxia but not in other organs (1). To directly investigate the role of CREB1 in the regulation of pulmonary vascular resistance we examined the responses of mice in which the α and Δ isoforms of CREB had been inactivated by gene mutation, leaving only the β isoform intact (CREBαΔ mice) (2). Wild type and CREBαΔ mice were exposed to hypoxia (10% O2) or normoxia (21% O2) for three weeks following which they were anaesthetized (sodium pentobarbital 60mg.kg-1, i.p) and exsanguinated. The lungs were removed and pulmonary vascular resistance (PVR) assessed using an isolated ventilated perfused preparation (n=5-13). After the baseline of measurements, a potent rho-kinase inhibitor Y-27362 (10-5 M) was added and the inhibitory effect of Y-27362 on PVR was assessed. Separate groups of lungs (n=8-9) were isolated and fixed for stereological assessment of the pulmonary vasculature as previously described (3). Gene expression (n=10-28) was investigated by real-time PCR. Protein expression (n=8) was measured by western blot. Expressions of CREB regulated genes, brain derived neurotrophic factor (BDNF), follistatin (FST), and tissue plasminogen activator (PLAT), was increased in the CREBαΔ mice (p<0.05, Mann-Whitney U). CREBαΔ mice had greater pulmonary vascular resistance than wild types, both basally in normoxia and following exposure to hypoxic conditions for 3 weeks (p<0.01, 2-way ANOVA). There was no difference in vasoconstrictor activity between CREBαΔ and wild type mice. Stereological analysis of pulmonary vascular structure showed characteristic wall thickening and lumen reduction in wild-type mice, with similar changes observed in CREBαΔ. CREBαΔ mice had larger lungs with reduced epithelial surface density (p<0.01, 2-way ANOVA) demonstrating increased pulmonary compliance, which caused increased vascular resistance. These findings show that CREB1 regulates homeostatic gene expression in the lung. Furthermore normal CREB1 activity is essential to maintain low pulmonary vascular resistance in both normoxic and hypoxic conditions and is required to maintain the normal alveolar wall structure and compliance. Interventions that enhance the transcriptional activity of CREB1 warrant further investigation as a potential therapeutic strategy in hypoxic lung diseases.