Abstract: We investigated the effects of ammonia stress on the enzyme activities in gills and digestive tract of juvenile mandarin fish (Siniperca chuatsi) with body mass of (13.31±0.49) g and body length of  (9.73±0.46) cm to reveal their physiological changes at  temperature of (22.0±0.5) ℃ and ammonia concentration of 48.65 mg·L⁻¹. The results show that the juveniles under acute ammonia nitrogen stress had undergone significant changes in the activity levels of Na⁺/K⁺-ATPase, respiratory metabolic enzymes and digestive enzymes. The Na⁺/K⁺-ATPase activity was significantly different from that of the control group, which first increased and then decreased with time. The activity of respiratory metabolism enzyme increased under stress. Digestive enzymes in the stomach and digestive tract were significantly affected too. AMS activity first decreased and then increased, while pepsin and LPS first  increased  and then decreased. The results suggest that ammonia nitrogen stress can cause a variety of impairments in fish body, including deterioration of antioxidant system, physiological metabolism, gill tissue, respiratory function and detoxification function. In order to keep impairment less, the digestive and anaerobic metabolism enzyme activities should be activated to higher level to produce energy for the body's resistance to stress response.