The purpose of this study is to observe the changes of pulmonary capillary permeability and various hemodynamic parameters before and after Positive End- Expiratory Pressure (PEEP) in Adult Respiratory Distress Syndrome (ARDS). Using a canine oleic acid induced ARDS model, we measured the pulmonary capillary permeability with a slope of lung:heart radioactivity ratio, hemodynamic parameters with Swan-Ganz catheter and blood gas tensions. 1) In normal and ARDS dogs, the PEEP didn't significantly influence the slope of lung:heart radioactivity ratio. But in ARDS group the slope index was increased compaired with that of control group (p〈0.05). 2) Also in ARDS group, PaO2 was significantly decreased, and PaCO2, PVCO2, MPAP, AaDO2, Qs/Qt were significantly increased compared with those of control group (p〈0. 05). 3) In normal dogs, the PEEP didn't influence blood pH or gas tension, AaDO2, Qs/Qt, or hemodynamics. 4) In ARDS dogs, however, the PEEP significantly increased PaO2, and decreased AaDO2, Qs/Qt (p〈0.05).