Abstract: AIM: To investigate the effect of bosentan on the expression of calponin in pulmonary arteries in rats with hypoxic pulmonary hypertension. METHODS: Forty healthy Sprague Dawley rats were randomly divided into four groups: control group, model group, placebo group and bosentan group, with ten rats in each group. The rats in the control group were kept under a normal environment for 6 weeks and the rats in the other groups were kept in a hypobaric chamber simulating the 5 000 m altitude 8 h/day for, respectively, 3 weeks, 6 weeks and 6 weeks. From the fourth week on, the rats in the model group were kept under a normal environment. For placebo group or bosentan group, sodium chloride (2 ml) or bosentan (100 mg/kg) was given for 3 weeks starting from the fourth week. The sixth week was taken as the end point. H/E stain was used to observe the morphological changes of pulmonary vessels and image analysis system was used to detect the relative medial thickness (RMT) of small- and medium-sized pulmonary arteries. Immunohistochemical technique was used to assess the localization and semiquantitation of calponin in pulmonary arteries. RESULTS: Compared to other groups, the pulmonary artery vessel wall in placebo group thickened and the vessel lumina was deflated. The pulmonary artery vessel wall and vessel lumina in bosentan group were restored to the status in the control group. Calponin was expressed in the pulmonary artery in all groups and the expression of calponin decreased with prolonged hypoxia. No significant difference was observed in the expression of calponin between bosentan group and control group. CONCLUSION: Bosentan promotes the synthesis of calponin in pulmonary arteries in rats with hypoxic pulmonary hypertension.