Abstract: Sprague-Dawley（SD） rat model of Pneumocystis carinii  pneumonia（PCP） was established by groin subcuta-neous injection with dexamethasone sodium phosphate 3.5 mg each twice a week for eight weeks. There were two groups：infected group（eighteen rats）  and normal control group（six rats）. Pathological changes in lung tissues were observed in the lung imprint after staining with Gomori methenamine silver（GMS）and in tissue sections after staining with hematoxylineosin（HE）. The activity of superoxide dismutase（SOD）and the content of lipidperoxide（LPO）in lung tissue homogenate were detected by spectrophotometric method. Results showed that in infected group more PC cysts were found in the lung imprint and typical pathological change observed in the lung section. SOD acitvity in infected group[（31.49±7.18） U/mgprot]decreased significantly compared with the control[（54.41±8.97） U/mgprot]（P<0.01），but LPO in infected group[（2.26±0.21） nmol/mgprot]was higher significantly than the control[（1.63±0.01） nmol/mgprot]（P<0.01）.