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It is well known that top predator organisms,such as marine mammals,accumulate very high concentration of Hg via trophic transfer.Level of total Hg(tHg)and methyl mercury(MeHg)has been reported to various organs of marine mammals although data from brain,which is the most important to assess the neurotoxicological effect,is relatively scarce due to difficulty of sampling.Once the relationship of Hg levels between brain and other organs is clarified,the ecotoxicological risk could be estimated using data of well investigated organs(e.g.,muscle or liver).Here we show the distribution of Hg among brain(HgB),muscle(HgM),and liver(HgL)using three dolphin species,i.e.,Harbor Porpoise(HP,n=39,Phocoena phocoena),Striped Dolphin(SD,n=34,Stenella coeruleoalba),and Meron-headed Whale(MW,n=24,Peponocephala electra).In all species,the highest concentration of tHgL among analyzed organs,low fraction of MeHg in liver(HP: 37±17%,SD: 4.4±4.9%,MW: 6.9±10.1%),and good correlation between tHgL and SeL(HP: r = 0.9731,SD: r = 0.9832,MW: r = 0.8952)were observed.These result indicates the occurrence of demethylation followed by formation of less mobile Se-Hg complex in liver.Correlation between tHgB and those in other organs showed different trends between larger dolphins inhabiting off shore(SD and MW)and smaller porpoise living near the coast(HP).tHgB in SD and MW showed better correlation to tHgL(r = 0.8952,r = 0.8278,respectively)than to tHgM(r = 0.6827,r = 0.6760,respectively),whereas tHgB-tHgM correlation(r = 0.8734)is slightly better than tHgB-tHgL(r = 0.8275)in HP.Since tHgM likely reflect recent exposure whereas tHgL represents integrated value of rather long time exposure,better correlation of tHgB-tHgL in large cetacean suggest brain has some sort of accumulation mechanism.Low fraction of MeHg in brain(SD: 39±19%,MW: 39±21%),and good correlation of tHgB-SeB in SD(r = 0.9850)and MW(r = 0.9167)suggest similar accumulation mechanism in liver.In contrast,tHgB-SeB of HP does not show significant correlation(r = 0.2886),suggesting less accumulation of Hg in brain.Good correlation of tHgB-tHgM than tHgB-tHgL of HP is consistent with this hypothesis.Larger lifetime exposure of Hg to large oceanic dolphins than small coastal porpoise possibly causes this difference.Hence,we conclude that HgL is better analog of HgB level in SD and MW,whereas HgM is acceptable choice in case of HP.