Hydroponics experiments were conducted to study the effects of atrazine stress (5,10,20,40 mg·L^-1) on O^-·₂ production rate,H₂O₂ content,activities of superoxide dismutase (SOD),catalase (CAT) and peroxidase (POD),malondialdehyde (MDA) content and plasma membrane permeability in leaves of Pennisetum hydridum,and the resistance and physiological mechanisms of P.hydridum to atrazine stress were discussed.The results indicated,plant samples were collected after 5,10,15 and 20 days.(1)Low level of atrazine stress (5,10 mg·L^-1) elevated O^-·₂ production rate and CAT activity in leaves of P.hydridum,and decreased H₂O₂ content,the activities of SOD and POD.With prolonging of cultivation time,the above indexes have returned to normal levels,but high level of atrazine stress (40 mg·L^-1) decreased H₂O₂ content,the activities of SOD,CAT and POD in leaves of P.hydridum continuously.(2)After 10 days,MDA content in leaves of P.hydridum treated by all levels of atrazine stress began to increasing.With the increase of atrazine concentration,the increasing amplitude of MDA content was significantly increased.All levels of atrazine stress showed no significant effects on relative permeability of plasma membrane.Therefore,P.hydridum probably regulates the activities of protective enzymes by signaling molecules such as reactive oxygen species,which could alleviate the injury caused by atrazine,and thus P.hydridum showed strong resistance to low level of atrazine stress (5,10 mg·L^-1).