Objective To investigate the effects of heparin on platelet derived growth factor (PDGF) in pulmonary artery smooth muscle cell (PASMC) of hypoxia - induced pulmonary hypertension rats. Methods We built the modle of hypoxia - induced pulmonary hypertension. Twenty - four male SD rats were randomly divided into three groups. Group A: normoxia and injected normal saline,group B: hypoxia cage and injected normal saline,group C: hypoxia cage and injected heparin. After three weeks, the right ventricular systolic pressure were measured by right -heart catheterization. Tested the content of PDGF in the lung by immunohistochemical test and analyse the images' gray scale by eomputor. Tested the content of PDGF mRNA in the lung by RT - PCR and display the results by the ratio of the strap of PDGF' s absorbance on that of GAPDH. Results The RVSP of both group A and group C were less than that of group B (P < 0. 01 ) ,and group C' s were higher than group A' s. In the immunohistochemical test, the group B' s gray scale were less than group C' s, and group C' s were less than group A' s. It was statistical significant when compared either two groups of the three (P < 0. 01 ) . The results of the RT- PCR test showed that the ratio of group B was higher than that of group C,and the ratio of group C was higher than that of group A. It was statistical significant when compared either two groups of the three (P < 0. O1 ) . Conclusion Pretreatment with heparin effectively prevents the formation of chronic hypoxic pulmonary hypertension. The mechanism may be relate to the inhibition of PDGF.