Thirty-day-old seedling of tomato (Lycopersicon esculentum Mill.) were treated with various Hg concentrations (0, 10, and 50 $mu$M) for up to 20 days, and the hypothesis that Hig induces oxidative stress leading to the reduction ofbiomass and chlorophyll content in leaves was examined. The accumulation of Hg in seedlings increased with external Hg concentration and exposure time, and Hg content in roots exposed to 50 $mu$M Hg for 20 days was about 27-fold higher than that in shoots. Furthermore, Hg exposure not only reduced biomass and chlorophyll levels in leaves but also caused an overall increase of endogenous H2O2, lipid peroxidation products (malondialdehyde), and antioxidant emzymes activities such as superoxide dismutase, catalase, and peroxidase in leaves and roots. Our results suggest that the suppression of growth and the reduction of chlorophyll levels in tomato seedlings exposed to toxic Hg levels may be caused by an enhanced production of active oxygen species and subsequent high lipid peroxidation.