Abstract: This study was aimed to investigate the effect of Hg2+ on oxidative damage and antioxidant capacity of grass carp （Ctenopharyngodon idellus）. The experimental fish were divided into control and treatment groups. The control group fish was raised in cultivated water， while the treatment group fish was exposed to 0.5 mg·L-1 Hg2+ for 24 hours first， and then transferred in cultivated water. Afterwards， they were sampled at 0 （24 h Hg2+exposure）， 5， 12， 21 d to test the superoxide dismutase （SOD）， catalase （CAT）， glutathion and peroxidase （GPx） activities as well as glutathione（GSH） and malondialdehyde （MDA） contents in their gill， hepatopancreas， spleen and kidney. It was shown that compared with the control group， the SOD， GPx activities （0， 5 d）， CAT activity （0， 5， 12 d） and GSH （5 d）， MDA （5， 12， 21 d） contents in gill in treatment group increased significantly（P<0.05）. In contrast， GPx activity （12， 21 d）， GSH and MDA contents （0 d） decreased significantly（P<0.05）. The SOD， CAT， GPx activities（0 d） and GSH （21 d），MDA （0，5 d） contents in hepatopancreas in treatment group rose sharply （P<0.05）. However， CAT activity（5， 12 d）， GSH （0， 5， 12 d） and MDA （12， 21 d）contents dropped significantly （P<0.05，P<0.01）. As for spleen， there was no significant difference on SOD， CAT and GPx activities between control and treatment group during the experimental period （P>0.05）. While， the GSH content （0 d） was lowered significantly（P<0.01），and the GSH （12 d） and MDA（0 d） contents were elevated remarkably（P<0.05）. The SOD （0， 5，12 d）， GPx（0 d） activities and GSH （12 d）， MDA （0 d） contents in kidney in treatment group were increased significantly （P<0.05）. Nevertheless，CAT （0，5，12 d） activity and GSH（0，5 d） content were decreased remarkably （P<0.05）. This test indicated that for grass carp， the SOD and CAT activities of organ tissues were recovered to the normal level. In hepatopancreas， GSH content exceeded the control group level （P<0.01）， and MDA content was significantly lower than that of control group （P<0.05）， but the GPx activity and MDA content in gill seemed unable to recover to the level of control group in the experimental period.

Key words: Hg²⁺ , Ctenopharyngodon idellus, antioxidant enzyme activities, GSH, MDA