The aim of this paper is to investigate the toxic effects of ammonia-N stress on the histopathological changes of gill, liver and kidney of juvenile Megalobrama amblycephala.The M.amblycephala, whose initial average weight was(14.27±0.01)g, were first exposed to ammonia-N for 96 hours and then obtained the 96 hours median lethal concentration.Based on this, the juvenile M.amblycephala were exposed to two ammonia-N levels:0.472 mg/L(control group)and 25 mg/L(experimental group), each ammonia-N level was respectively sampled at 0, 6, 12, 24, 48 h, then the remainder of them were moved into aerated tap water for a 96 h post-exposure recovery.The results showed that the 96 h LC₅₀ of ammonia-N were found to be 56.492 mg/L.Fish exposed to different ammonia concentrations displayed histopathologic alteraions in the gills, liver and kidney.Gill tissues displayed hyperemia, epithelial hyperplasia, chloride cell hyperplasia, telangiectasis, epithelial necrosis and desquamation, epithelial and oedema, lamellar fusion curling of secondary lamellae as well as shortening of secondary lamellae.The liver showed hypertrophy of hepatocytes, cloudy degeneration, karyolysis, cellular peripheral nucleus, nuclear hypertrophy, hepato cellular vacuolation, dilatation in sinusoids, congestion, hydropic degeneration, necrosis, cellular outline indistinguishable and cytolysis;hypertrophied epithelial cells of renal tubules, reduction of space inside renal tubules, expansion of space inside, Bowman's capsule, contraction of the glomerulus, necrosis in tubular epithelium, cloudy swelling of epithelial cells of renal tubules, tubular necrosis, lymphocytic cell infiltration, renal tubules epithelial cells vacuolation, hydropic degeneration and hyperemia were observed in kidney.The results indicate that the severity of lesions clearly differed among organs with the liver showing the most extensive damages followed in order by the gills and kidney.The damages of gill, liver and kidney increase with increasing exposure time, in the meanwhile, the fish showed the self-regulation ability, however, it could not fully recover after a 96-hour post-expourse recovery and the kidney showed the worst recovery ability.