Abstract: Heavy metal bioaccumulation of fish in rivers and lakes or reservoirs are discussed in relation to background values of elements in fish in Changjiang River water system and the Fengtan Reservoir, Hunan Province.The carp in lakes or reservoirs has greater concentrations than in rivers for Hg, Cd and Pb, and has less concentrations than in rivers for As and Se.F value, which is the fish mercury content in reservoir compared to the content in river within a same river basin, is greater than 1, but smaller than 1 for As and Se.The regression analysis shows that there is a good correlationship tetween F and ratio values of reservoir drainage area(A) to its runoff(Q):F=2.5096-0.48047A/Q (r =- 0.795, n=12)This shows that the smaller values of A/Q, the greater extent of mercury in fish.Comparing calculated F values to measured values, their relative errors are 2.1-40.4%,average relative error is 12.6%.The regression equation is confirmed by results investigated in the Fengtan Reservoir.Measured in field F values are 1.29-1.92 with the mean value 1.60, and the relative error 23.1%.It is well filled with the predicted F value.On the basis of mis regression equation, it is predicted that the bioconcentration of mercury in fish will increase after construction of the Three Gorge Reservoir, contents of mercury will be 1.6-2.0 bine higher than present contents, and there might be a possibility that fish mercury contents might exceed the concentration of 0.3mg/kg which is the limit content of the Food Hygiene Law of the People's Republic of China (tentative).In the high mercury background zone of the low Wujiang River, this will produce the severe impacts on fishery development and community health.