Abstract: To study the inflammatory reaction and lung injury in pneumocystis carinii pneumonia （PCP），PCP was induced in Sprague-Dawley rats by the administration of cortisone acetate as immunosuppressant for 8-12 weeks.The Rats were divided into four groups，Group Ⅰ： normal control （n= 6），consisted of healthy rats without the steroid treatment ； group Ⅱ:negative control （n= 6），rats free from lung infection； group Ⅲ ： bacterial pneumonia （n= 11）； group Ⅳ （n= 14） ： PCP. Cells in the bron-choalveolar lavage fluid （BALF） were counted and differentiated，as well as the amounts of total protein （TP）, albumin （ALB） and the activities of alkaline phosphatase （ALP）, lactate dehydrogenase （LDH）and type Ⅳ collagenase （matrix metallo-proteinases, MMP-2，MMP-9） were detected. And compared the differences in inflammatory reaction and lung injury between PCP and bacterial pneumonia . The total cell count in the BALF of PCP was significantly increased （P＜0.01），but was lower than that in the bacterial pneumonia group. ALB in the BALF of PCP （622.5±484.0μg/ml） was increased as compared with the negative controls （35. 8±16.2 yg/ml，P<＜0.01）.The activities of ALP and LDH in PCP （217.0±81.4u/L，20.8±8.2 u/dl respectively） were also higher than those in the negative control （P＜0. 01）. The activities of Type IV collagenase were increased in PCP （P＜0.001）. The inflammatory response in PCP differed from that found in bacterial pneumonia. The permeability of the alveolar capillary membrane and the lung injury were significantly increased in PCP.