AIM:To investigate effect of hypoxia on the expression of proliferating cell nuclear antigen(PCNA) a nd phenotype of cardiac fibroblasts(CFs). METHODS: The purif ied cardiac fibro blasts were cultured and divided randomly into there groups :control group, mode rate hypoxia(MH) group and severe hypoxia(SH) group. After 72 h,MTT method was u s ed to investigate the proliferation of CFs, and the ultrastructure of fibroblast s were observed with transmission electron microscopy The expression of PCNA a n d ?-actin in cardiac fibroblasts were measured by the means of immunohistochemi s try and laser scanning confocal microscopy, respectively. RESULTS: MTT A 490 nm value of MH group was significantly higher than that of control group by (18 4?25 0)% ( P