Aquaporin-1 (AQP1) is a water channel highly expressed on tissues with rapid O₂ transit as erythrocytes, microvessel endothelium, and alveolar epithelial cells. Besides its classical function as water channel AQP1 has been also demonstrated to contribute to gas permeation (CO₂ and NO) trough the cell membrane. In the central nervous system expression of several AQPs have been found altered by exposure to hypoxia and subsequent reoxygenation. Experiments of our laboratory demonstrated that lung AQP1 is markedly up-regulated in animals exposed to hypoxia. Therefore we decide to investigate the molecular mechanisms underlying hypoxia regulation of AQP1 expression. Transcriptional regulation was studied by transient transfections into mouse endothelial cells with a 1277 bp 5' proximal AQP1 promoter-luciferase construct. Hypoxia incubation produced a dose and time dependent induction of luciferase activity. After 24h of incubation at 21% O₂ (normoxia, Nx) or at different levels of hypoxia (Hy), the ratio hypoxia vs normoxia (Hy/Nx) was about 1 for either the promoterless (pXP2) construct or untransfected cells. However, when the AQP1 promoter-Luc construct was transfected the Hy/Nx ratio increased to 1.26±0.33, 4.50± 1.53 and 13.13±3.33 at oxygen levels of 6%, 3% or 1% respectively. Hy/Nx ratio of luciferase activity rose to 1.64±0.81 (6%), 14.98±1.46 (3%) and 16.69±3.79 (1%) when the hypoxia incubation was done for 48h. Values represent mean ± standard error of 6 experiments. Accordingly with these results the promoter region of AQP1 analyzed here contains three putative HIF binding domains that may participate in the hypoxia regulation of the AQP1 gene.