Abstract: Objective To observe the expresson of GSTO 1 in lymphocytes of the people with and without arsenic exposure.Methods A group of subjects(n=6)was selected from slightarsenism patients,severe arsenism patients,arsenism area residents,and nonarsennism area residents,respectively.The expression of glatathione S-transferase omega1(GSTO 1) mRNA in lymphocytes was detected with real-tmie quantitative polyme rase chain reaction(RT-PCR).The concen trations of inorganic arsenic(AiS),monomethyl arsenic acid(MMA),dmiethylarsenic acid(DMA)and total arsenic acid(TAs)in urine were detected with AFS-9130 and SAP 10.Results iAs,MMA,DMA,TAs,primary methylation index(PMI),secondary methylation index(SMI)of urine among slight and severe arsenism patients and the controls in the arsenism district were significantly higher than that of nonarsenic exposure group.The expressions of GSTO1 mRNA of the arsnism with servere and moderate symptoms(P₅₀=1.96),slight symptoms(P50 =1.31)and the controls(P50=1.23)in arsenism area were significantly correlative with the expression of GSTO 1 mRNA and MMA%(r=0.206,P=0.445).Conclusion The expresson of GSTO 1 mRNA is not associated with arsenic methylation.The arsenic poisoning is associated with the overexpression of GSTO 1 mRNA because of arsenic exposure,which reduces quinquevalent arsenics into trivalent arsenics.