An acute salinity stress trial was conducted to estimate the effect of dietary magnesium level on serum osmolality,serum ions and ATP enzyme in gill filaments of Japanese seabass Lateolabrax japonicus.Four semipurified diets were formulated containing 0.413 g(D1),1.042 g(D2),1.577 g(D3),1.991 g(D4)magnesium(as magnesium sulphate)per kilogram diet to feed triplicate groups of fish［initial average weight(30±0.5)g］in freshwater circulation system for 50 days.Then all fish were transferred from freshwater to seawater to collect the serum and gill filaments at 0,1,3,6,12,24 h,respectively.The results showed that dietary magnesium levels significantly affected the levels of serum ions(Na+,Cl-,K+,Mg2+),serum osmolality and the activities of Na+/K+ATPase(NKA)and Ca2+/Mg2+ATPase(CMA)in gill filaments at 0 h(P<0.05).The serum osmolality,serum Mg2+and NKA of gill filaments increased with the increase of dietary magnesium levels.The levels of serum Mg2+ and osmolality,NKA and CMA activities in gill filaments of fish fed D4 were significantly higher than those fed the other diets(P<0.05),which indicated the Japanese seabass fed D4 was in a state of high intensity osmotic pressure regulating.During the salinity stress periods,the contents of serum Na+ and Cl-increased with the time extension of salinity stress,while the levels of serum osmolality,K+,Ca2+ and Mg2+,NKA and CMA activities fluctuated with stress duration from 0 to 24 h.NKA and CMA activities of fish fed D1 were significantly lower than those of other groups,which indicated it would reduce the sensitivities of NKA and MKA in gill filaments to salinity stimulation for Japanese seabass fed low magnesium level diet for 50 d.The results suggested that in order to reduce energy consumption for higher activities of NKA and CMA in gill,dietary magnesium level should be lower than 1.991 g/kg for Japanese seabass cultured in freshwater,however,high dietary magnesium levels improve NKA and CMA activities in gill,which could be beneficial to quick adaptation high salinity environment for Japanese seabass transferred from low salinity water into high salinity water.