The effects of increasing concentrations of copper on the growth, ultra-structure and on certain biochemical parameters of water lettuce (Pistia stratiotes L.) were investigated under controlled conditions in the nutrient solutions containing increased copper sulfate concentrations ranging from 0 to 100 microM. Copper treatment for 12, 18 or 24 h resulted in inhibition of roots and leaves dry biomass. Atomic absorption spectrometry analysis of roots and leaves showed that copper accumulation increased with increase in concentration and duration of metal treatment. It is seen that copper resulted in increased production of hydrogen peroxide and superoxide radical in both roots and leave cells, showed a significant change after 24 h of treatment. Also, the significant decrease in the contents of total protein and photosynthetic pigments was observed. The antioxidant enzymes, viz., peroxidase (POX, E.C.1.11.1.7), catalase (CAT, E.C.1.11.1.6) and superoxide dismutase (SOD, E.C.1.15.1.1) showed significant variation with the increase in lipid peroxidation. Increasing trends was observed in levels of ascorbate and glutathione. The rapid inducibility of some of these enzymes are useful early and sensitive indicators of heavy metal toxicity. The results demonstrated that exposure to elevated concentration of Cu had a remarkable effect on the biochemistry and physiology, induced oxidative stress in water lettuce characterized by the initiation of lipid peroxidation that inhibited growth and disintegration of major antioxidant systems.