Inoculated 2 month old Japanese black pine( Pinus thunbergii )and slash pine( P.elliottii )seedling with pine wood nematode( Bursaphelenchus xylophilus PWN),the experiment studied changes of active oxygen metabolism in the pine seedlings and the relations between the changes and disease development.Activities of superoxide dismutase(SOD)and catalase(CAT)reduced gradually in Japanese black pine seedlings after infection by PWN,and increased in infected slash pine seedlings at the early stage,but decreased at the later stage.Activity of peroxidase(POD),contents of H 2O 2 and malondialdehyde(MDA)increased progressively in the seedlings.The rising of contents of H 2O 2 and MDA were coincident with the speeds of seedling wilting.The results showed that the balance of the host’s active oxygen metabolism was disturbed,the rapid death of the seedlings was associated with active oxygen accumulation in the seedling tissues after infection by PWN.The increase of the speed of membrane lipid peroxidation might be the major factor accelerating the death of the host’s cells and stimulates the development of the disease.