Ammonia (i.e. the total of NH(3) and NH(4)(+)) has been one of the main causes of the decline of macrophytes in fresh water. In order to study the effects of ammonia toxicity, plants of the submersed macrophyte Vallisneria natans (Lour.) Hara were treated with various concentrations of NH(4)Cl (0.1, 0.4, 1.2, 2 and 2.8mM) for 4 days or with 2mM NH(4)Cl for different lengths of time (12h, 1, 2, 4 and 8 days). The toxic effect and oxidative stress caused by NH(4)Cl resulted in a reduction of total chlorophyll (chlorophyll a and b) and an increase in the levels of reactive oxygen species (ROS) O(2)(-) and H(2)O(2), with an increased concentration of NH(4)Cl and duration of exposure. Meanwhile, weak chlorosis and water-soaked symptoms were observed in older leaves exposed to 2.8mM NH(4)Cl for 4 days. The activity of superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase (POD), ascorbate peroxidase (APX) and glutathione reductase (GR) was up-regulated in leaves treated with 1.2, 2 and 2.8mM NH(4)Cl for 4 days or with 2mM NH(4)Cl for 1, 2 and 4 days, when compared with controls. Among these enzymes, the activity of superoxide dismutase, ascorbate peroxidase and glutathione reductase was significantly up-regulated in plants treated with 0.4mM NH(4)Cl for 4 days, while they were down-regulated at 4 and 8 days from their peak values in leaves treated with 2mM NH(4)Cl. The content of ascorbic acid decreased significantly in leaves treated with 0.4-2.8mM NH(4)Cl for 4 days or with 2mM NH(4)Cl for 2-8 days. The content of total glutathione (tGSH; reduced and oxidized glutathione) increased in leaves treated with NH(4)Cl at 0.4, 1.2 and 2mM for 4 days or with 2mM NH(4)Cl at 1, 2 and 4 days, while tGSH was decreased below the level of controls by treatment with 2.8mM NH(4)Cl for 4 days or to the level of controls by treatment with 2mM NH(4)Cl for 8 days. However, the content of malondialdehyde (MDA) decreased with increased concentration of NH(4)Cl and duration of exposure. Results from staining with 3,3'-dimethoxybenzidine (DAB) further indicated that the level of H(2)O(2) and the activity of guaiacol peroxidase increased significantly in plants exposed to 2mM NH(4)Cl for 4 days. These results suggested that ammonia exerted its toxic effect on the growth of V. natans (Lour.) Hara, at least in part, by induction of oxidative stress and inhibition of photosynthesis. The decrease in the content of malondialdehyde is discussed.