Abstract: Effects of exogenous abscisic acid(ABA) on the activities of some enzymes,such as superoxide dismutase(SOD),catalase(CAT),ascorbate peroxidase(APX),peroxidase(POD),and some other physio-biochemical indices,such as O₂^.- generation rate,chlorophyll,soluble protein and proline contents,in leaves of Potamogen crispus with or without Cd²⁺ stress were observed.Results show that Cd²⁺,4 mg·L^-1 in concentration,decreased activities of SOD,CAT and APX,but increased O₂^.- generation rate,activities of POD and proline content,thus leading to loss of chlorophyll and soluble protein.Exogenous ABA obviously enhanced activities of SOD,CAT and APX,but inhibited O₂^.- generation and activity of POD.In addition,ABA also increased the loss of chlorophyll and soluble protein triggered by Cd²⁺ stress,and proline content,thus enhancing the ability of Potamogen crispus to stand Cd²⁺ stress.The most suitable concentration of ABA is defined to be in the range of 2.5-5 μmol·L^-1.