Abstract: Objective To investigate DNA strand breaks induced by NaAsO₂ in different phases and different dosages in vivo.Methods Mice are treated throughs.c.injection with NaAsO₂ at a dose of 0.4,2 and 10mg/kg.After injection mice are sacrificed at the end of 1,3 and 6 hours.DNA strand breaks are detected in the splenocyte,thymocyte,marrow cells and liver cells throug h SCG E(Single Cell Gel Electrophoresis).Results After different dosages treatment it is found that 2 and 10mg/kg NaAsO₂ could induce DNA damage not only in the frequency of comet cell but also in the migration of DNA breaks in cells from spleen,thymus and bone marrow.During the research of different time-course study,1 hour after NaAsO₂ treatment splenocyte appears DNA strand breaks most apparently.The peak of DNA strand breaks in thy mocyte and marrow cell apperes 3 hours after treatment.After 6 hours treatment the DNA strand breaks fall down and accesse to normal level in different organs.NaAsO₂ induces DNA strand breaks in liver cell in our dosage and time scope.Conclusion Two mg/kg and 10mg/kg NaAsO₂ could induce DNA strand breaks in cells from spleen,thymus and bone marrow in vivo,but different cell type mighthave different reaction to NaAsO₂.