The changes of microscopic structures and the number of immune cells of spleen of the Ricefield Eel(Monopterus albus)were investigated after the fish were exposed separately to different concentrations of mercury ions(Hg²⁺) and the Chromium ions(Cr⁶⁺) for 1,2,4,and 8 days.The results showed that the splenic capsule was very thin and the trabeculae could not be found in the control group.The splenic parenchyma was composed of white pulp and red pulp.The lymphocytes of white pulp aggregated into groups.The splenic nodule could seldom be observed,but periarterial lymphatic sheath could be observed in the spleen.The red pulp was made of splenic cord and splenic sinus.In the spleen,the ellipsoids opened to the white and red pulps.The melano-macrophages gathered to from a centre.Comparing with the control,histological structure of fishes exposed to two kinds of heavy metal ions displayed similar changes.Along with increasing heavy metal ion concentration and lengthening treatment time,melano-macrophage centre gradually increased in the number and distribution,then reduced.The number of melano-macrophages first increased,and then reduced.Lymphatic tissue gradually loosed,and its arrangement was sparse and chaotic.The lymphocyte border was unclear,and its number first increased,and then reduced,showing a degenerative tendency.The change of granulocytes was similar to that of lymphocytes.The red blood cells were massively damaged,and the sinusoidwas expanded.