Objective:To investigate the effects of calcium-activated chloride (ClCa) channels on proliferation of pulmonary artery smooth muscle cells(PASMCs) in rats under chronic hypoxic condition. Methods:The cultured PASMCs were placed under normoxic and chronic hypoxic conditions:The cells were observed by light and electron microscope; The cell cycles were observed by flow-cytometry; Immunocytochemistry staining was used to detect the expressions of PCNA, c-fos and c-jun of PASMCs; Cytoplasmic free Ca2+ con-centration ([Ca2+]i) in PASMCs was investigated by fluorescent quantitation using fluorospectrophotometer. Results:The PASMCs were contractile phenotype under normoxic conditions. Observation by transmission electron microscope: In kytoplasm of contractile phenotype cells, myofilament bundles were abundant and the content of cell organs such as Golgi’s bodies were rare. The PASMCs were synthetic phenotype under chronic hypoxic condition. There were increased free ribosomes, dilated rough endoplasmic reticulums, highly developed Golgi complexes, decreased or disappeared thick filaments and dense body in kytoplasm of synthetic phenotype cells. After NFA and IAA-94, the situations were reversed The number of S+G2M PASMCs were significantly increased in chronic hypoxic condition; The NFA and IAA-94 were shown to significantly decrease them from (28.6±1.0)% to (16.0±1.6)% and the number of G0G1 PASMCs significantly increased from (71.4±1.9)% to (83.9±1.6)% (P < 0.01). In chronic hypoxic conditions, the expression of proliferating cell nucleus antigen was significantly increased; The NFA and IAA-94 were shown to significantly decrease it from (81±6)% to (27±7)%(P < 0.01). The expression of c-fos and c-jun were significantly increased in chronic hypoxic conditions; The NFA and IAA-94 were shown to significantly decrease them from 0.15±0.02, 0.32±0.05 to 0.05±0.01, 0.12±0.05, respectively (P < 0.01); Under chronic hypoxic conditions, [Ca2+]i was increased; The NFA and IAA-94 decreased it from (281.8±16.5)nmol/L to (117.7±15.4)nmol/L(P < 0.01). Conclusion:Hypoxia initiated the change of PASMCs from contractile to synthetic phenotype and increased proliferation of PASMCs. NFA and IAA-94 depressed cell proliferation by blocking ClCa channels in hypoxic condition. These may play an important role in proliferation of PASMCs under chronic hypoxic conditions.