Background: For a long time, the vasoactivity of pulmonary veins has been debated. Increasing evidences about the role of pulmonary veins to the total pulmonary vascular resistance has been particularly well supported by studies associated to development of fetal and neonatal pathology. In contrast, the vasoactivity of pulmonary veins in adult mammalians has been more controversial and largely unexplored. Nevertheless, the alterations of the vascular tone of pulmonary veins are believed to play an important role during the development of cardiovascular diseases including Pulmonary Arterial Hypertension (PAH). In the lung, nucleotides are released from the cytoplasm of many cells including endothelial, smooth muscle and epithelial cells under physiological and pathological conditions. Particularly, release of ATP and UTP has been found elevated under certain pulmonary diseases. This extracellular ATP and UTP binds to P2Y2/4 receptors, widely expressed in blood vessels, attributing a pivotal role in the control of vascular tone. However, there are no studies on either the effects of ATP and UTP on small intrapulmonary vein (SPV) contraction or the mechanisms that couple purinergic signalling to PAH. Here we have used ‘living’ lung slices and phase-contrast video microscopy to investigate, for the first time, purinergic- dependent dynamic changes in SPV contraction in PAH rats. Methods and Results: These studies were approved by the ethics committee of Fac. of Medicine, University of Chile (CBA#0614 FMUCH). Lung slices (150μm thick), from healthy and Monocrotaline (MCT)-induced PAH Sprague Dawley rats (200gr), in a vibratome were performed.. ATP and UTP-induced SPV contraction was recorded using phase contrast video microscopy. Statistical differences (p<0.05) were performed using non-parametric tests. After 21 days of a single subcutaneous injection of MCT, (60mg/Kg) the rats develop PAH, including right ventricle hypertrophy. Also, in PAH rats there was an exacerbated venous constriction in response to UTP (EC50=6,9±2,45μM) versus healthy rats (EC50=19±5,01μM). Similarly, ATP-dependent vasoconstriction was strongest in PAH (EC50 =14,3±1.9μM) in comparison with healthy rats (EC50=28.6,3±1.1μM). ATP and UTP-induced SPV contraction was strongly inhibited by Suramin, a non specific antagonist of purinergic receptors. Significant P2Y2 inhibition (41.5±9.0%) with ARC118925XX was predominant only in healthy rats, but not PAH rats, suggesting a main role of P2Y4 in PAH. The presence of P2Y2 and P2Y4 receptors in co-localization with smooth muscle cells was demonstrated by indirect immunofluorescence. Conclusion: These results suggest a novel mechanism involving P2Y4 receptor in exacerbated vasoconstriction observed in PAH. The study of purinergic therapies to improve quality of life of PAH patients is promising.