Glutathione concentrations were measured in rat bronchoalveolar lavage fluid (BALF) obtained from normal rats and rats exposed to a fraction of inspired oxygen (FiO2) of 0.8 for up to 5 days. We also perturbed rat lung glutathione concentrations by administering the compound diisopropylidene acetone (phorone) to a separate group of animals and correlated changes in BALF glutathione with changes in lung tissue glutathione. We found that reduced glutathione is present in normal rat BALF but glutathione disulfide is extremely low. Increases in lung tissue glutathione concentration and in BALF glutathione concentration occurred after 5 days of exposure to hyperoxia. Animals treated with phorone exhibited decreases in lung glutathione concentration two hours after dosing and increases in lung glutathione concentration 24 hours after dosing. Rat BALF obtained from phorone-treated animals at 2 or 24 hours after administration revealed that changes in BALF glutathione concentrations reflected changes in lung tissue glutathione concentration. The presence of glutathione in lung lavage fluid suggests that the compound could be playing an extracellular role in the lung, either as an antioxidant or as a coenzyme for other glutathione-related enzymatic reactions.