The juveniles of hybrid tilapia （ Oreochromis niloticus x Oreochromis areus） were exposed to ananonia-nitrogen （N） （0, 2.5, 5, 10 and 20 mg/L） for 0, 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 signifi- candy with extension of stress time （P 〈 0.05）. The total antioxidant capacity （T-AOC） and activity of antioxidase in liver were significantly affected. The activi- ty 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 ex- perimental conditions, non-specific immunity of tilapia was affected by ammonia-N stress, and the impact was increased with increased concentration and extension of time.
