To assess its response to the herbicide, tribenuron-methyl, samples of Nasturtium officinale were exposed to 0, 0.01, 0.05, 0.1, and 0.5 $mg;L^{-1}$ of tribenuron-methyl for 1, 2, 4 and 7 days. The influence of this herbicide on the relative growth rate, electrolyte leakage, lipid peroxidation, photosynthetic pigmentation, protein content, and performance of anti-oxidant enzymes, such as superoxide dismutase (SOD), catalase, and ascorbate peroxidase (APX), was examined. The results indicated that tribenuron-methyl, applied at 0.5 $mg;L^{-1}$, affected plant growth negatively. It also was determined that chlorophyll a is the most responsive photosynthetic pigment to tribenuron-methyl exposure. Under stress conditions, the anti-oxidant enzymes were up-regulated compared to the control. The SOD activity was significantly stimulated, while the activity of APX was inhibited. A significant correlation was found between lipid peroxidation and SOD activity. The exposure period and herbicide concentration had significant effects on the biological responses against tribenuron-methyl stress. These results may be useful for clarifying the effect of herbicides on non-target aquatic plants.