A hydroponic experiment was conducted to study the growth, manganese (Mn) accumulation, lipid peroxidation, H2O2 concentration, and antioxidant system of Phytolacca americana L. exposed to different concentration Mn. With increasing Mn concentration in the medium, the plant Mn content increased significantly, and the Mn accumulation was in the sequence of leaf > stem > root. Comparing with the control, low concentration (5 mmol x L(-1)) Mn promoted the plant growth, decreased the leaf H2O2 concentration, and had less effects on the leaf malondialdehyde (MDA) content, while high concentration (> or = 10 mmol x L(-1)) Mn led to a remarkable increase of leaf H2O2 and MDA contents, indicating an evident oxidative damage occurred in leaves. The activities of ascorbate peroxidase, dehydroascorbate reductase and glutathione reductase and the content of reduced ascorbate increased with increasing Mn concentration, while the SOD activity was inhibited significantly at 5 mmol x L(-1) of Mn but enhanced at > or = 10 mmol x L(-1) of Mn. The activities of catalase and peroxidase and the content of reduced glutathione increased at 5-10 mmol x L(-1) of Mn but dropped markedly at 20 mmol x L(-1) of Mn. All the results suggested that the Mn-induced oxidative damage and Mn accumulation might be responsible for the growth inhibition of P. americana plants at high Mn exposure, and the increase of antioxidative enzyme activities and low molecular antioxidant contents was, at least partly, contributed to the Mn tolerance and hyperaccumulation of P. americana. However, due to their different Mn concentration-dependent change modes, these antioxidants played different roles in the Mn tolerance of P. americana.