Chlorine (Cl₂) is highly reactive oxidant gas used extensively in a number of industrial processes. Exposure to high concentrations of Cl₂ has resulted in acute lung injury which may either resolve spontaneously or progress to acute respiratory failure. Presently, the pathophysiologic sequelae associated with Cl₂-induced acute lung injury in conscious animals, as well as the cellular and biochemical mechanisms involved, have not been elucidated. We exposed conscious Sprague Dawley rats to Cl₂ gas (either 184 ppm or 400 ppm) for 30 min in environmental chambers and then returned them to room air. At one hr post exposure, rats showed evidence of arterial hypoxemia, respiratory acidosis, increased levels of albumin, IgG and IgM in bronchoalveolar lavage fluid (BALF), increased BALF surfactant surface tension and significant histological injury to their airway and alveolar epithelia. These changes were more pronounced in the 400 ppm exposed rats. Concomitant decreases of ascorbate (AA) and reduced glutathione (GSH) were also detected in both BALF and lung tissues. In contrast, heart tissues AA and GSH content remained unchanged. These abnormalities persisted 24 h post exposure in rats exposed to 400 ppm Cl₂. Rats injected systemically with a mixture of AA, deferoxamine and N-acetyl-cysteine prior to exposure to 184 ppm Cl₂, had normal levels of AA and lower levels of BLAF albumin and normal Pa_O2 and Pa_CO2 values. These findings suggest that Cl₂ inhalation damages both airway and alveolar epithelial tissues and that resulting effects were ameliorated by prophylactic administration of low molecular weight antioxidants.