Objective To examine the effects of hypoxia on endothelial-to-mesenchymal transition of porcine pulmonary arterial endothelial cells ( PAECs) .Methods The porcine PAECs were divided into a normoxia group and a hypoxia group. The cells in two groups were exposed to normoxic or hypoxic condition for 1,4, and 7 days respectively. The immunofluorescence,Western blot and RT-PCR were used to detect the protein and mRNA expressions of VE-cadherin and α-SMA. Results The porcine primary PAECs formed typical monolayer of cobblestone appearance on normoxia condition, and had a spindle-shaped appearance on hypoxia condition. Immunofluorescence results showed that these PAECs expressed mesenchymal cells specific marker of α-SMA. With the hypoxic time prolongation, the ratio of transdifferentiated smooth musclelike cells from PAECs was gradually increased ( P lt; 0. 01) . Western blot assay demonstrated that the expression level of VE-cadherin protein and mRNA was reduced gradually, but the expression level of α-SMA protein and mRNA was increased. Conclusion Hypoxia can induce endothelial-to-mesenchymal transition, which may be involved in the development of a variety of diseases.