To verify the physiological mechanism of exogenous nitric oxide (NO) on ameliorating Al toxicity in rice,we planted cultivar ‘Zhefu 802’ in hydroponic solutions,and then detected the reactive oxygen species and antioxidase activity.The results showed that:(1)0.05 mmol/L Al significantly inhibited root growth,caused Al and callose accumulation in root tips,and increased the contents of reactive hydrogen peroxide (H₂O₂) and superoxide anion freebase (O^-·₂).Pretreatment the rice seedlings were applied with 0.1 mmol/L sodium nitroprusside in the presence of Al (SNP,NO donor) increased relative root elongation and NO content by 34.96% and 12.86%,respectively,reduced Al and callose contents by 83.04% and 31.93%,indicating that NO partially alleviated Al toxicity which might be caused by the rising levels of endogenous NO.(2)The exogenous NO decreased H₂O₂ and O^-·₂ contents by 15.43% and 12.93%,respectively,and significantly enhanced SOD,POD and CAT activities in rice root tips.In contrary,the ameliorating effect was reversed by the addition of NO scavenger (carboxy-PTIO,cPTIO) in the presence of Al.These results showed that exogenous NO played an important role in maintaining cell membrane structure stability by regulating reactive oxygen metabolism under Al toxicity,thereby reducing the Al damage to rice roots.