To evaluate the biological effects of wastewater samples containing heavy metals, the effects of metal Cd (II) and As (V) were studied on Spirodela polyrrhiza L. The plants were exposed at metal concentrations 0.1, 0.5, 1, 2 μM of Cd (II) and 1, 5, 10, 20 μM of As (V) for a period of 1, 4, 7 d (day) alone and in combination of both. Plants accumulated 1855 mg kg−1 dw (dry weight) Cd and 1230 mg kg−1 dw As after 7 d in alone, whereas it was 885 mg kg−1 dw Cd and 865 mg kg−1 dw As in combination. The toxicological parameters such as fresh biomass, photosynthetic pigments, and total protein contents increased up to 2 μM of Cd (II) after 1 d and 10 μM of As (V) after 4 d with respect to control (Hormesis effect), followed by gradual decline at higher concentrations and duration. In case of Cd (II) a maximum decrease of 58% in protein content, 62% in fresh biomass, and 78% in total chlorophyll was observed at 2 μM, whereas, with As (V) 38% decrease in protein content, 34% in fresh biomass, and 52% in total chlorophyll was shown at 20 μM after 7 d. The metal tolerance strategy against metal induced reactive oxygen species adopted by the plants was investigated with reference to nonprotein thiols (NP-SH), cysteine, and ascorbic acid. The results of combined treatment revealed reduced toxicity at the level of fresh biomass, protein content, and chlorophyll; however, the amount of nonenzymatic antioxidant did not significantly (P = 0.172) increase as compared to alone treatment. Finally, it was concluded that due to high metal accumulation coupled with defense potential, the plant appears to have a potential for its use as phytoremediator species of aquatic environments. © 2007 Wiley Periodicals, Inc. Environ Toxicol 22: 539–549, 2007.