The effects of perfluorooctane sulfonate(PFOS) exposure on hepatic Cu/Zn-SOD and stress-related gene expression in swordtails(Xiphophorus helleri) were studied in order to screen the sensitive biomarkers induced oxidative stress in vivo.We cloned and analyzed the swordtail fish Cu/Zn-SOD cDNA of full sequence.Swordtails were randomly divided into five groups, with one control group and four experimental groups(3.5, 7.0, 14.0 and 28.0 mg/L PFOS).Parallel experimental groups were also established.The Cu/Zn-SOD, HSP70-1, HSP70-2 and GST mRNA expression levels in fish livers were determined after 24 h, 48 h, 96 h, and 14 d exposure.The X.helleri Cu/Zn-SOD cDNA was cloned, which had 794 bp nucleotides, encoding 154 amino acids.The comparison among the similarity nucleotide sequence and amino acid sequence of Cu/Zn-SOD in other related fishes showed that nucleotide and amino acid sequence similarity were 77%-83% and 79%-88%, respectively.After exposure to high concentrations of PFOS, a variety of stress-related genes in X.helleri liver changed in different degrees.The expression of Cu/Zn-SOD mRNA in the dose-effect was not significant by RT-PCR, and with no correlation between the SOD activity and the expression of Cu/Zn-SOD mRNA.It may be other types of SOD gene that play more important role under the PFOS exposure.With the exposure time to increase continuously, Cu/Zn-SOD gene changes were consistent with the SOD activity and the expression of HSP70-1 mRNA.Thus HSP70 by increasing the level of swordtail fish SOD protected the body from oxidative damage.HSP70-2 mRNA was more sensitive than the levels of HSP70-1 mRNA expression.The changes of HSP70-2 were not relevant with Cu/Zn-SOD mRNA expression.The underlying mechanism needed further study.GST mRNA expression was constantly increasing, which is related with the Nrf2 signal transduction pathways, and it may also be a form of defense in fish when PFOS entered the body.Swordtails in vivo experiments showed that PFOS could induce oxidative stress in the liver.The expression of GST mRNA and HSP70-2 mRNA could act as sensitive biomarkers of PFOS exposure.