The accumulation of arsenic (As) and physiological responses of Lemna minor L. under different concentration (0, 1, 4, 16 and 64 mu M) and duration (1, 2, 4 and 6 days) of two species As, NaAsO2 and Na2HAsO4 center dot 7H(2)O, were studied in hydroponics. The accumulation of both As species depended on As concentration and exposure duration. The highest accumulation of As was found as 17408 and 8674 mu g g(-1), for plants exposed to 64 mu M of As(III) and As(V), respectively, after 6 days. Two-way ANOVA analyses indicated that, for plants exposed to arsenite (As(III)), exposure duration had a greater effect than concentration on As accumulation. Conversely, exposure concentration had a greater effect on As accumulation in plants exposed to arsenate (As(V)). Arsenic exposure levels, approaching 16 mu M for As(III) and 64 mu M for As(V), did not significantly affect EC values. Beyond these exposure concentrations, EC values increased in a manner that depended on duration. Significant effect of As(III) on lipid peroxidation was observed at 1 mu M application whereas, this effect started to be significant after an exposure to 16 mu M As(V). For both As(III) and As(V), photosynthetic pigment levels slightly increased for the first day with respect to the control, followed by a gradual decline at higher concentrations and durations. An increase in protein content and enzyme activity was observed at moderate exposure conditions, followed by a decrease. Significant positive correlations were determined between accumulated As and ion leakage and lipid peroxidation. Negative correlations were found between accumulated As and total chlorophyll and protein content. Our results suggested that exposure duration and concentration had a strong synergetic effect on antioxidant enzyme activity. The findings of the present study may be useful when this plant is used as a phytoremediator in arsenic-polluted water.