Increased pulmonary vascular resistance in Pulmonary Hypertension (PH) is caused by vasoconstriction and obstruction of small pulmonary arteries by proliferating vascular cells. In analogy to cancer, subsets of proliferating cells may be derived from endothelial cells transitioning into a mesenchymal phenotype. To understand phenotypic shifts transpiring within endothelial cells in PH, we injected rats with alkaloid monocrotaline to induce PH and measured lung tissue levels of endothelial-specific protein and critical differentiation marker Vascular Endothelial (VE)-cadherin. VE-cadherin expression by immonoblotting declined significantly twenty-four hours and fifteen days post injection to rebound to baseline at thirty days. There was a concomitant increase in transcriptional repressors Snail and Slug along with a reduction in VE-cadherin mRNA. Mesenchymal markers α-smooth muscle actin and vimentin were upregulated by immunohistochemistry and immunoblotting and α-smooth muscle actin co-localized with endothelial marker Platelet-Endothelial-Cell-Adhesion-Molecule-1 (PECAM-1) by confocal microscopy. Apoptosis was limited in this model, especially in the 24-hour time-point. In addition, monocrotaline resulted in activation of protein kinase B/Akt, endothelial nitric oxide synthase (eNOS), Nuclear Factor (NF)-B and increased lung tissue nitrotyrosine staining. To understand the etiologic relationship between nitrosative stress and VE-cadherin suppression, we incubated cultured rat lung endothelial cells with endothelin-1, a vasoconstrictor and pro-proliferative agent in PAH. This resulted in activation of eNOS, NF-B and Akt, in addition to induction of Snail, downregulation of VE-cadherin and synthesis of vimentin. These effects were blocked by eNOS inhibitor L-NAME. We propose that transcriptional repression of VE-cadherin by nitrosative stress is involved in endothelial- mesenchymal transdifferentiation in experimental PH.