This study was designed to investigate the pathogenesis of chlorine gas (Cl2) induced acute lung injury and oedema. Isolated blood-perfused rabbit lungs were ventilated either with air (n=7) or air plus 500 parts per million (ppm) of Cl2 (n=7) for 10 min. Capillary pressure, measured by analysing the pressure/time transients of pulmonary arterial, venous and double (both arterial and venous) occlusions, was unchanged in both groups. In Cl2-exposed lungs, the fluid filtration rate increased from -0.228+/-0.25 to 1.823+/-1.23 mL min(-1) x 100 g(-1) (p<0.001) and the filtration coefficient increased from 0.091+/-0.01 to 0.259+/-0.07 mL x min(-1) x cmH2O(-1) x 100 g(-1) (p<0.001). No changes were observed in the control lungs. The extravascular lung water/blood-free dry weight ratio was 8.6+/-1.6 in the Cl2 group and 4.0+/-0.5 in the control group (p<0.001), confirming that the increase in lung weight was related to accumulation of extravascular fluid. Although the alveolar flooding by oedema is explained, in part, by the Cl2-induced epithelial injury, our results suggest that Cl2 exposure induces acute lung injury and oedema due to an increased microvascular permeability.