Abstract: The effects of water stress on leaf membrane lipid peroxidation and endogenous protective systems in longan (Dimocarpus longana Lour.) young seedlings were studied.Under water stress,the relative water content (RWC) decreased and the electrolyte leakage of plasma membrane increased;there was a negative linear correlation between RWC and electrolyte leakage;the activities of superoxidase dimutase (SOD) and peroxidase (POD) and the contents of malondialdehyde (MDA) increased and there were positive linear correlations with electrolyte leakage;the contents of reduced glutathione (GSH) and soluble protein and the activity of ascorbate peroxidase(AsA-POD)gradually decreased when water stress gradually increased and there were negative linear correlations with electrolyte leakage and the content of MDA;the activities of glutathione reductase (GR) and catalase (CAT) increased at first,then decreased.