Ozone (O3) has been assumed to be the most phytotoxic air pollutant due to significant damage to the plants.But some researchers suggest that hydrogen peroxide (H2O2)and organic peroxides such as methyl hydroperoxide (MHP) maybe a new reason caused damage of plant.The "peroxides theory" is also supported by the results that high concentrations of hydrogen peroxide and organic hydroproxides were observed in the declined forest, and compared with exposure to 100 ppb O3, exposure to 2-3 ppb peroxides + 50 ppb O3 induced more reduction in biomass of Japanese radish.To investigate the effects of ambient-level gas-phase peroxides concurrent with O3 on foliar injury and photosynthesis in forest plants, we exposed Japanese white birch and beech to clean air, 50 ppb O3, 100 ppb O3, and 2-3 ppb peroxides t 50 ppb O3 in outdoor chambers.Compared with exposure to 100 ppb O3, exposure to 2-3 ppb peroxides + 50 ppb O3 induced greater damage in foliar injury and net photosynthetic rates both in white birch (Fig.1, Table 1) and beech.The pattern of foliar injury induced by 2-3 ppb peroxides + 50 ppb O3 is differed from those occurring with O3 exposure alone.Compared with exposure to 100 ppb O3, significant decrease in net photosynthetic rates were found both in white birch and beech exposure to 2-3 ppb peroxides + 50 ppb O3.The reasons induced the decrease in net photosynthetic rates in white birch exposure to 2-3 ppb peroxides + 50 ppb O3 are different from that in beech.These results indicate for the first time that sub-ppb peroxides t 50 ppb O3 can cause more severe damage to forest plants than 100 ppb O3, and that not only O3, but also peroxides,could be contributing to the forest plant damage observed in Japans remote mountains areas.