Rabbit fish(Siganus guttatus)(67.76?26.12)g were reared under different salinities(fresh water,salinity 5,10,20 and control group(the local seawater))to study the response of survival,growth and antioxidant defense system to salinity.The rabbit fish which were fed twice everyday were held in 500 L conical plastic tank with 95 cm in height and 95 cm in diameter for 40 days and three parallels each group.After 40 d,six fish each group were taken randomly to collect the samples of the gill,liver,kidney and muscle.The results showed that the abnormal phenomenon was not found.The survival rate of salinity 5,10,20 and the control group during the experimental period was 100%.However,the abnormal phenomenon of the rabbit fish in fresh water was found(the food consumption was reduced)on the sixth day.They fasted from the 7th day and began to die from 9thd.Feed was recovered from the 13th day.However,death continued.The fish in fresh water died out after 27 d.Specific growth rate(SGR)in different salinities did not show significant differences.The body weight in salinity 10 was significantly higher than that in salinity 5 and 20.However,no significant difference was observed between salinity 10 and the control group.The total length in salinity 5 was significantly lower than other groups.In order to study the response of antioxidant defense system to salinity,the activity of superoxide dismutase(SOD),catalase(CAT),superoxide anion radical,hydroxyl radical of rabbit fish were measured.The activities of SOD,CAT,superoxide anion radical and hydroxyl radical in gill,liver,kidney,muscle of rabbit fish,reared under salinity 10,20,returned to normal levels(the level of control group)after 48 d.No significant differences were observed among each group of each index.However,the activities of SOD and superoxide anion radical in gill of rabbit fish under salinity 5 were significantly higher than that under salinity 10,20 and the control group.The activity of SOD in liver of rabbit fish was the highest among all groups,and followed by kidney and gill,muscle was the lowest and significantly lower than that in other tissues(P<0.05);the activity of CAT in liver of rabbit fish among all groups was the highest and significantly higher than that in other tissues(P<0.01),and followed by gill and kidney,muscle was the lowest;the activity of superoxide anion in liver of rabbit fish among all groups was the highest,and significantly higher than that in other tissues(P<0.01),and followed by muscle,kidney and gill were the lowest;the activity of hydroxyl radical in kidney of rabbit fish among all groups was the highest and significantly higher than that in other tissues(P<0.05);and followed by gill,liver and muscle were the lowest.The results showed that salinity can affect the activity of antioxidative enzymes,but did not affect the distribution.