Abstract: Objective To explore the distribution and metabolism of arsenic speciation in the liver and brain of the mice exposed to iAs at the early life.Methods Arsenic species in both liver and brain of mother mice and their offspring were determined by hydride generation-atomic absorption spectrometry(HG-AAS).Results The levels of iAs,monomethylarsonic acid(MMA) and dimethylarsinic acid(DMA) in the liver and the levels of iAs and DMA in the brain of mother mice increased along with the concentration of iAsv in drinking water.In the liver of the offspring mice,the levels of iAs increased from postneonatel day(PND) 15,and MMA increased from PND 21.The levels of DMA on PND 10(high exposure group:0.020±0.005) and on PND 15(high exposure group:0.031±0.012) were the lowest,and levels of DMA on PND 21(high exposure group:0.239±0.076) were significantly higher than those on PND 0 and 35.In the brain,the levels of iAs on PND 21(high exposure group:0.088±0.042) were the highest and MMA were undetectable at the early developmental stages.The levels of DMA on PND 10 and 15 were the lowest in the brain,and then increased along with the development.Conclusion The results suggest that the breast barrier could prevent iAs and DMA from entering the milk of mother mice efficiently.Only DMA could transit the mature blood brain barrier and mature blood-brain barrier could effectively prevent iAs from entering brain tissue.