To understand the effect of enhanced UV-B radiation and low-energy N+ ion beam radiation on the response of photosynthesis, antioxidant enzymes, and lipid peroxidation in rice seedlings, Oryza sativa was exposed to three different doses of low-energy N+ ion beam and enhanced UV-B alone and in combination. Enhanced UV-B caused a marked decline in some photosynthetic parameters (net photosynthetic rate, transpiration rate, and stomatal conductance) and photosynthetic pigments, whereas it induced an increase in hydrogen peroxide (H2O2) accumulation, the rate of superoxide radical production, and the content of malondialdehyde (MDA). Enhanced UV-B also induced an increase in the activity of antioxidant enzymes (superoxide dismutase [SOD], peroxidase (POD), and catalase [CAT]) and some nonenzymatic antioxidants such as proline. Under the combined treatment of enhanced UV-B and low-energy N+ ion beam at the dose of 3.0 x 10(17) N+ cm(-2), the activity of antioxidant compounds (SOD, POD, CAT, proline, and glutathione), photosynthetic pigments, and some photosynthetic parameters (net photosynthetic rate, transpiration rate, and stomatal conductance) increased significantly; however, the MDA content, H2O2 accumulation, and rate of superoxide radical production showed a remarkable decrease compared with the enhanced UV-B treatment alone. These results implied that the appropriate dose of low-energy N+ ion beam treatment may alleviate the damage caused by the enhanced UV-B radiation on rice.