Paraquat (PQ) is a contact herbicide that acts specifically on Photosystem II by blocking the normal electron flux and NADPH reduction reaction. As a consequence, many active oxygen species (AOS) are generated and result in deleterious effects on biological structures. In plants, the biochemical mechanisms involved with PQ resistance are still unknown.  We have used sugarcane as a physiological model since the antioxidant enzymatic system has not been reported in literature and this specie has economical importance.   We analyzed the physiological effects of PQ in leaves 24 and 48 hours after exposure to different concentrations (0, 2, 4, 6 e 8 mM) of Paraquat (metil violagen). PQ treatments showed oxidative damages in the lipid fraction as revealed by progressive increase in TBARS formation, indicating that ROS generated by PQ might cause damages in cell membranes. Interestingly, SOD specific activity was stimulated by PQ being dose and time dependent. The most evident SOD isoform induced by PQ, in both exposition times, was the SOD Cu/Zn, which is located preferentially in chloroplasts. It is important to remark that even in almost lethal concentrations of PQ the chloroplast SOD activity still remains expressed similarly to the control plants. The ascorbate peroxidase (APX) activity was stimulated by low levels of PQ but it decreased abruptly after 4 mM treatment. Under very toxic levels of herbicide the maintenance of SOD activity was not accomplished by APX activity wich strongly suggests that H₂O₂ might be responsible for the oxidative damage caused in sugar cane leaves. Therefore, it is possible to assume that APX and other chloroplast peroxidases are not able to scavengers the H₂O₂ formed by the PQ-induced SOD activity. In addition, SDS-PAGE and 2-DE analysis showed induction of three major proteins (MM aprox. 70, 40 and 20 KD respectively) in the treatment of 4 mM PQ. These proteins might be related to others defense mechanisms against the herbicide injury. The upholding of chloroplast SOD and APX proteins in sugarcane leaves treated with PQ should be detailed investigated since it can be a molecular target for breeding programs in order to obtain plants with an acquired resistance to PQ or others herbicides with model action in the photochemical system.