AIM: To investigate the effect of hypoxia-preconditioned human umbilical cord mesenchymal stem cell-derived exosomes (hUCMSC-Exos) on pulmonary vascular endothelial-mesenchymal transition (EndMT) in hypoxic pulmonary hypertension (HPH). METHODS: (l) Primary hUCMSCs were isolated and cultured by tissue adhesion method, and hUCMSC-Exos were extracted by ultrafiltration and identified. (2) Twenty-four SPF male SD rats were randomly divided into normoxia (N) group, hypoxia (H) group, hypoxia+normoxic hUCMSC-Exos group and hypoxia+hypoxia-preconditioned hUCMSC-Exos group, with 6 rats in each group. The rats in H group and intervention groups were placed in a cabin that simulated the hypoxic environment at an altitude of 5 000 m, and normoxic hUCMSC-Exos, hypoxia-precon-ditioned hUCMSC-Exos or equivalent volume of PBS were injected through the tail vein on the 3rd, 5th, 7th, 10th and 14th days in hypoxia environment. After 21 d of modeling, the right ventricular systolic pressure (RVSP) and right ventricular hypertrophy index (RVHI) of the rats were detected, and the pathological changes of lung tissues were observed by HE staining. (3) After starvation for 12 h, human pulmonary arteriole endothelial cells (HPAECs) were randomly divided into normoxic control (N-Con) group, hypoxic model (H-Con) group, hypoxia+normoxic hUCMSC-Exos group and hypoxia+hypoxia-preconditioned hUCMSC-Exos group. The migration ability and tube formation ability of HPAECs were detected by Transwell assay and tube formation experiment. The expression of CD31 and α-smooth muscle actin (α-SMA) in HPAECs was detected by immunofluorescence double-staining. The protein levels of CD31, VE-cadherin, α-SMA and vimentin in pulmonary vessels and HPAECs were assessed by Western blot. RESULTS: (1) The HPH rat model was successfully established after 21 d of hypoxia, and EndMT occurred in pulmonary vessels. Compared with N group, the levels of RVSP, RVHI, percentage of vascular wall area (WA%) and percentage of vascular wall thickness (WT%) in H group were significantly increased (P<0.01), pulmonary vascular wall thickening and the protein levels of CD31 and VE-cadherin were significantly decreased (P<0. 01), while the protein levels of α-SMA and vimentin were significantly increased in pulmonary vessels (P<0.05 or P<0.01). Compared with H group, the RVSP, RVHI, WA% and WT% (P<0.01) were significantly decreased (P<0.05 or P<0.01), and pulmonary vascular remodeling was attenuated after normoxic or hypoxia-preconditioned hUCMSC-Exos intervention. After hypoxia-preconditioned hUCMSC-Exos intervention, HPH pul-monary vascular remodeling and EndMT formation were significantly inhibited. (2) After 48 h of hypoxic treatment, the migration, tubule formation and EndMT of HPAECs were induced. Compared with H-Con group, cell migration and tube formation were significantly decreased after hypoxia-preconditioned hUCMSC-Exos intervention (P<0. 01). The protein levels of CD31 and VE-cadherin were increased, while the protein levels of α-SMA and vimentin were decreased (P<0.05 or P<0.01). CONCLUSION: Hypoxia-preconditioned hUCMSC-Exos attenuate the formation of HPH pulmonary vascular remodeling by inhibiting pulmonary vascular EndMT. [ABSTRACT FROM AUTHOR]