【Objective】In order to provide theoretical basis for breeding， production and application of alkalinity-to-
lerant tilapia strains， the toxicity of carbonate alkalinity to Oreochromis niloticus and corresponding mechanism of alkalinity tolerance were studied. 【Method】O. niloticus individuals were divided into four groups and directly transferred into carbo-
nate alkalinity treatments（4.0， 6.0， 8.0 and 10.0 g/L NaHCO3） from freshwater for acute stress. The serum pH， ammonia concentration and relative expression of carbonic anhydrase（CA） gene and glutamine synthetase（GS） gene in kidney were determined using pH meter， UV spectrophotometer and quantitative real-time PCR post-transfer at 0.5， 1.0， 4.0， 6.0， 9.0， 12.0， 18.0， 24.0， 36.0 and 48.0 h， respectively. 【Result】In all treatment groups， the serum pH and ammonia concentration changed significantly after 1.0 h（P<0.05）. In 4.0 and 6.0 g/L NaHCO3 treatment groups， no dead fish was found within 48.0 h， and the serum pH， ammonia concentration， relative expression of CA and GS genes showed “up-peak-down” change trend. In 8.0 g/L NaHCO3 treatment group， dead fishes were found in succession after 9.0 h， and the corresponding serum pH and ammonia concentration was 8.11 and 486.28 μmol/L， respectively， all fishes were dead after 12.0 h. In 10.0 g/L NaHCO3 treatment group， dead fishes were found earlier than those in 8.0 g/L NaHCO3 treatments， and the serum pH and ammonia concentration were 8.14 and 365.89 μmol/L after 1.0 h， respectively， all fishes were dead after 4.0 h. 【Conclusion】O. niloticus can tolerate a serum pH of up to 8.10 and ammonia concentration of at least 525.66 μmol/L. O. niloticus can adapt to alkalinity water to a certain degree in less than 6.0 g/L NaHCO3 though regulation of pH and ammonia， and CA and GS are involved in ion transportations and nitrogen metabolism under alkalinity stress.