The biological system for defensing peroxidation damage could reflect the early influence of toxic pollutants on the water environment

which could be regarded as the early warning index used in the biological monitoring.The findings related to the variation tendency of the effects of methyl chlorpyrifos（1 μg/L） to some specific index and major enzymatic activity in crucian’s liver were described in this study

following the expose time during the different periods in 29 days

including growth index

liver index

antioxidant enzymes activity（superoxygen dismutase and glutathione reductase）

and transaminase activity.From the integrated survey results obtained

there exist protection modes of mutual competition and compensatory coorperation in crucian.Glutathione reductase was recommended as a sensitive biomarker for its marked variation during the exposure periods

and applied in monitoring and evaluating the effect of pollutants to aquatic organisms.