Leaf discs were excised from spinach leaves (Spinacia oleracea L.) and floated in phosphate buffer (pH 6.8) containing paraquat solutions (0, 0.1, 1.0, 10, 20, 50 and 100 ppm), and incubated in the growth chamber under 5,500 lux illumination at $25^{circ}C$ for 24 hr. Treatment with paraquat caused the formation of malondialdehyde (MDA), an indicator of lipid peroxidation in leaf discs. When 1.0, 10, 20, 50 and 100 ppm of paraquat solutions were applied to leaf discs, the contents of MDA were increased by 63, 86, 100, 140 and $150\%$ of the level without paraquat treatment respectively. 1.0, 10, 20, 50 and 100 ppm of paraquat treatments reduced the amounts of ascorbic acid in leaf discs by 23, 35, 38, 42 and $56\%$ of the level without paraquat treatment, respectively. Activities of superoxide dismutase (SOD) in leaf discs of 1.0, 10, 20, 50 and 100 ppm of paraquat treatments were decreased by 23, 42, 48, 61 and $70\%$ of the level of SOD in non-treated group, respectively. The results suggest that paraquat may cause peroxidation of membrane lipid in spinach leaves as a result of paraquat-induced destruction of physiological defense against oxygen phytotoxicity.