To understand the response of fish antioxidant defense and neural system to mercury exposure,studies were conducted in laboratory between 26 and 29 ℃ on the acute toxicity and exposed effects on antioxidant enzymes and AChE activities in the young crimson snapper,Lutjanus erythropterus Bloch,of an average weight of(4.57?2.02) g and body length(4.95?0.79) cm.Using static renewal method,the results showed that the safe concentration of mercury chloride for the young crimson snapper was 0.040 mg/L.The enzymes activities were evaluated under four sublethal concentrations(0.000 5 mg/L,0.002 5 mg/L, 0.005 mg/L and 0.025 mg/L Hg² ).The tissues(gill and liver)were tested for SOD,MDA activity and the brain for AChE after 6 h,12 h,24 h,48 h and 96 h exposure,respectively.The results showed that hepatic SOD activities had significant increase on 6 h exposure(P<0.05).With the increase of mercury concentration,the hepatic SOD activities were inhibited or induced,which showed the disturbance of fish metabolism.And the branchial SOD activities were also significantly elevated.The variation of MDA content exhibited that the gill was impaired on 6 h exposure,while the hepatic tissue was detected to be impaired on 12 h.The AChE activities in the brain had no significant decrease(P>0.05)on 6 h exposure.However,it decreased with the induction rate of 36% on 12 h exposure.When the fish were exposed longer time,the inhibition rates were 28%-38%,14%-24% and 12%-17% on 24 h,48 h and 96 h exposure in concentrations respectively.