OBJECTIVE: To explore the effects of vinyl chloride monomer( VCM) on the expression of mir-122 in the liver tissue and serum of rat. METHODS: Specific pathogen free adult male SD rats were randomly divided into 4 groups with 24 rats in each group: the control group,the low-,medium- and high-dose groups. The 4 groups were intraperitoneally injected with VCM exposure levels at 0,5,25 and 125 mg / kg body weight 3 times per week,respectively. Eight rats per group were randomly selected and sacrificed after 6,8 and 12 weeks. Small RNA( < 200 nt) in the liver tissue and total RNA in the serum were isolated and purified. Real-time fluorescent quantitative polymease chain reaction technique was used to detect their expressions of mir-122. RESULTS: Compared with the control group and the low-dose group at the same time points,the relative expressions of mir-122 in the liver tissue of rats in the medium- and high-dose groups were significantly increased after exposed to VCM for 6 weeks( P < 0. 05),and then were significantly decreased after exposed to VCM for 12 weeks( P < 0. 05). The relative expressions of mir-122 in the liver tissue of rats in the medium- and highdose groups were decreased with VCM exposure time( P < 0. 05). Compared with the control group at the same time points,the relative expressions of mir-122 in serum of rats in the three dose groups were significantly increased after being exposed to VCM for 8 weeks( P < 0. 05),and then were significantly decreased after being exposed to VCM for 12 weeks(P < 0. 05). Among the three dose groups,the relative expressions of mir-122 in serum of rats after 12-week exposure were lower than those after 6-week and 8-week exposure in the same group respectively( P < 0. 05). CONCLUSION:s The VCM exposure significantly changes the expression of mir-122 in the liver tissue and serum of rat,which suggested that mir-122 may be one of the effect biomarkers for VCM exposure.