To understand the physiological change processes in adaptation to salinity and alkalinity water,Oreochromis niloticus were divided into four groups and directly transferred into four salinity-alkalinity treatments(A:salinity 10,1 g/L NaHCO₃;B:salinity 10,2 g/L NaHCO₃;C:salinity 15,1 g/L NaHCO₃;D:salinity 15,2 g/L NaHCO₃)from freshwater,and the acute changes of serum osmolality,serum Na⁺,K⁺,Cl^- concentration and mRNA expression of Na⁺-K⁺-ATPase(NKA)and carbonic anhydrase(CA)in gill of different treatments were observed at 0,6,12,24,36,48,72 and 96 h post-transfer,respectively.Results showed that the changes of serum osmolality,serum Na⁺,K⁺,Cl^- concentration,and NKA,CA gene mRNA expression were relevant to the salinity-alkalinity stress levels.Meantime,these parameters shared a similar “up-peak-down” change trend.The serum osmolality in B,D treatments reached the peak at 24 h,while A,C treatments at 36 h.Serum Na⁺,K⁺,Cl^- concentration in all treatments reached the peak at 24 h.The highest NKA gene mRNA expression in B,D treatments was detected at 36 h,while 24 h in A and C treatments.The highest CA gene mRNA expression in B,C and D treatments was observed at 24 h.These results showed O.niloticus could adapt to salinity-alkalinity water to a certain degree,and NKA and CA were involved in ion transportations and osmolality balance under salinity-alkalinity stress.