AIM: To study the expression level of heme oxygenase -1 and -2 in pulmonary arterial smooth muscle cell(PASMC) treated with carbon monoxide(CO). METHODS: (1) The expression of HO in cultured rat PASMC was determined by immunohistochemistry stain. (2) The levels of HO mRNA and protein were evaluated by reverse transcription- polymerase chain reaction(RT-PCR) and Western blot. RESULTS: (1) PASMC cytoplasm showed yellow (++-+++) in hypoxia group, the buffy color(++++) in CO group as cells were stained by HO-1 antibody. (2) The expression level of HO-1 mRNA (1.25?0.37) in hypoxia group was higher than that of normal group (0.12?0.04). It was much higher (3.52?0.68) in CO group, than that of hypoxia group. (3)The content of HO-1 protein (1.07?0.15) in hypoxia 48 h group was higher than that of hypoxia 24 h group(0.52?0.04). It was higher(3.65?0.43) in CO group than that of hypoxia group.It was highest in CO 48 h group. CONCLUSION: Low density of CO and hypoxia enhanced the expression of HO-1 mRNA and protein in PASMC when cell were treated in hypoxia or CO,indicating that HO-1 has regulatory effect on PASMC during hypoxia.