OBJECTIVE: To investigate the changes of pulmonary shunt fraction (Qs/Qt), alveolar dead space (V Dalv), gas exchange and respiratory mechanics in the septic acute respiratory distress syndrome (ARDS) and acute lung injury (ALI). METHODS: Ten anaesthetized and ventilated young pigs were given an infusion of endotoxin for an experimental observation of 180 min to induce ARDS/ALI. The mean pulmonary artery pressure (MPAP), cardiac output (CO), respiratory compliance (Crs), end tidal CO2 (ET CO2), and single breath test for CO2 (SBT CO2), PaO2 and PaCO2 were recorded. Cardiac index (CI), Qs/Qt, V Dalv and ET CO2/PaCO2 were calculated. RESULTS: Endotoxin induced ARDS/ALI decreased in CI, Crs, PaO2, ET CO2/PaCO2, and increased in MPAP, Qs/Qt, V Dalv (P<0.05). CONCLUSIONS: Qs/Qt and V Dalv in combination with haemodynamics, respiratory mechanics and arterial blood gases are of great clinical importance in the diagnosis of ARDS/ALI and analysis of pathophysiological events.