An important environmental concern associated with intensive fish culture is the production of ammonia, which is formed through ammonification of organic matter like unconsumed food and feces. The object of this study was to evaluate the effects of ammonia-N stress on the gill Na⁺/K⁺-ATPase, microstructure and serum physiological-biochemical indices of juvenile black carp, Mylopharyngodon piceus. The black carp, whose initial average weight was (7.00 ? 0.14) g, were transferred off the freshwater and exposed to different ammonia-N levels: 0 mg/L (low, control), 10 mg/L (middle) and 20 mg/L (high) NH₄Cl, each ammonia-N level was randomly triplicate and sampled at 0, 6, 12, 24, 48 and 96 h, respectively. The results indicated that the gill Na⁺/K⁺-ATPase activities in the middle and high ammonia-N treatment groups decreased gradually at first and reached the minimum at 12 h, 6 h, respectively, then increased gradually and reached the maximum at 48 h, and returned to the same level as that of control group after 96 h. Gill microstructure observation showed that chloride cell in middle ammonia group increased at 12 h; pavement cells fell off partly at 24 h; chloride cells vacuolated partly, blood capillary fractured and red blood cell overflowed at 96 h. But in high ammonia group, chloride cell increased at 6 h; pavement cells fell off partly at 12 h and largely at 24 h; After 96 h, blood capillary fractured on the base region of the lamellae and congestion, pillar cell disorganized of arrangement. Serum cortisol and glucose concentration in ammonia-N treatment group increased gradually at first 12 h, and then decreased gradually and returned to the same level as that of control group after 48 h. The activities of total superoxide dismutase (SOD) in ammonia-N treatment group increased significantly at first 6 h (P<0.05), and SOD activity in the middle ammonia-N treatment group returned to the same level as that of control group after 12 h, but SOD activity in the high ammonia-N treatment group was still significantly higher than that of control group after 96 h (P<0.05). Serum catalase activity decreased at first 6 h and then increased and reached the maximum at 12 h, and then decreased and recovered to the same level as that of control group after 48 h. Total antioxidant capacity (T-AOC), glutathione (GSH) and alanine transaminase (ALT) activities in am-monia-N treatment group decreased gradually at first 24 h, and malondialdehyde (MDA) concentration showed converse trends. After 96 h, GSH and ALT activities in ammonia-N treatment group returned to the same level as that of control group, however, the MDA concentration in ammonia group was significantly higher than that of control group and T-AOC activity in high ammonia group was significantly lower than that of control group. These results indicated that the ammonia stress could seriously disturb the antioxidant system at short-term ammonia-N stress, but fish showed the self-regulation ability , however, the antioxidant capacity decreased and gill structure was damaged after long-term ammonia-N stress.