Oxidative stress induced by oligogalacturonides in Lycopersicon esculentum MilI. cv. Rio grande cultured cells was investigated. The treatment triggered a rapid synthesis ofH20 2, which was detected within the first 2 hours in the extracellular medium, reaching a 4.5-fold increment after 6 h. There was a sequential and concerted activity ofthe antioxidant systems, in which at the beginning, the activity of superoxide dismutase increased to induce a hyper-oxidative cellular state. Mterwards, the activities of H20 2-consuming enzymes, ascorbate peroxidase, catalase and guaiacol peroxidases, increased to return the cells to the same redox levels of those not receiving the treatment.