The juveniles of hybrid tilapia( Oreochromis niloticus × Oreochromis areus) were exposed to ammonia-nitrogen( N)( 0,2. 5,5,10 and 20 mg / L) for0,12,24,48 and 72 h to evaluate the effect of ammonia-N stress on their non-specific immunity. Results show that the activity of serum lysozyme decreased significantly with extension of stress time( P < 0. 05). The total antioxidant capacity( T-AOC) and activity of antioxidase in liver were significantly affected. The activity of T-AOC and total superoxide dismutase( T-SOD) of fish exposed to ammonia-N were initially decreasing then increasing( P < 0. 05). Activities of catalase( CAT) and glutathione peroxidase( GSH-Px) were correlated with concentrations of ammonia-N. Fish exposed to lower concentrations( 2. 5 mg/L or 5 mg/L)showed decreased CAT activity within 24 h( P < 0. 05),while those exposed to higher concentrations( 10 mg / L or 20 mg / L) initially showed increased then decreased activity of CAT. Except for the highest concentration groups,fish exposed to ammonia-N showed inductive activity of GSH-Px( P < 0. 05). Under the experimental conditions,non-specific immunity of tilapia was affected by ammonia-N stress,and the impact was increased with increased concentration and extension of time.
