Abstract: This study was aimed to investigate the effect of Hg²⁺ on antioxidant function of grass carp, Ctenopharyngodon idella. The experimental fish were divided into control and treatment groups. The control group fish were fed in normally cultivated water, and the treatment group fish were exposed to water containing Hg²⁺ concentration of 0.07,0.22,0.37,0.52 mg·L^-1, respectively. The superoxide dismutase (SOD), glutathione peroxidase (GPx) activities as well as glutathione (GSH) and malondialdehyde (MDA) contents in their gill, hepatopancreas, spleen and kidney were tested on 1, 5, 12, 21 d after treatment. The results were as follows: compared with the control group, during the experimental period, for the tested organ tissues, the SOD, GPx activities(0.52 mg·L^-1)and GSH (0.37, 0.52 mg·L^-1) contents decreased significantly (P<0.05), while MDA contents (0.37, 0.52 mg·L^-1) increased remarkably(P<0.05). When exposed to Hg²⁺ for 1 d, the SOD, GPx activities (0.07, 0.22, 0.37 mg·L^-1 groups) and GSH contents (0.07, 0.22 mg·L^-1 groups) in the tested organ tissues rose in different degree, however, MDA content in gill fell dramatically (P<0.05). When exposed to Hg²⁺ for 5 d, the SOD activities in gill, spleen, kidney (0.37 mg·L^-1), the GPx activities in gill, hepatopancreas, kidney (0.37 mg·L^-1), the GSH contents in the tested organ tissues (0.37 mg·L^-1) reduced apparently (P<0.05), and the same changes took place in the SOD, GPx activities (0.37 mg·L^-1) when exposed to Hg²⁺ for 12 d. When exposed to Hg²⁺ for 21 d, the SOD , GPx activities in the tested organ tissues (0.22, 0.37 mg·L^-1) and GSH contents in the spleen and kidney(0.22 mg·L^-1) dropped significantly (P<0.05), nevertheless, MDA contents in the tested organ tissues (0.22 mg·L^-1) increased greatly (P<0.05). It could be concluded that for grass carp, the antioxidant function was promoted by low Hg²⁺ concentration in short time exposure, but inhibited by high Hg²⁺ concentration in long time exposure. The experiment showed that the stress responses of the organ tissues and the antioxidant components were not the same under Hg²⁺stress, this might be due to the characteristics of the organ tissues and the components, but also reflected the complexity of Hg²⁺ toxic effects.

Key words: Hg2+, Ctenopharyngodon idella, antioxidant function