Bioaccumulation and toxicity of copper was evaluated on Potamogeton pusillus L. The effect of copper (5-100 microg L(-1)) applied for several days was assessed by measuring changes in the chlorophyll's, phaeophytin's, malondialdehyde, electrical conductivity, glutathione peroxidase (GPX), glutathione reductase (GR) and guaiacol peroxidase (POD) activities. Plants accumulated copper with a maximum of 162 microg g(-1) dw after 7-days exposure at 100 microg L(-1), however most of the metal was accumulated after 1-day exposure. The toxic effect caused by Cu was evident by the reduction of photosynthetic pigments, increase of malondialdehyde and electrical conductivity. P. pusillus shows Cu-induced oxidative stress by modulating antioxidant enzymes like GPX, GR and POD. Antioxidant enzymes activity increased significantly after exposure to 40 microg L(-1) during 24 h, followed by a drop at longer times. Thus, P. pusillus is proposed as a good biomonitor for the assessment of metal pollution in aquatic ecosystems.