The effects of inhaled endotoxin on airway caliber and on airway vascular permeability were examined in awake guinea pigs. A single 3 hour exposure to 10 - 100 µg/m³ endotoxin produced a dose-related decrease in specific airway conductance by the end of exposure (-22% for 10 µg/m³ and -34% for 100 µg/m³). This change in conductance was not observed in control animals exposed to an aerosol of normal saline. Immediately following exposure to endotoxin or saline, airway vascular permeability was measured by quantitation of the extravasation of Evans blue dye into the trachea, mainstream bronchi, and extrapulmonary hilar bronchi. Endotoxin exposure caused a dose-dependant increase in Evans blue dye extravasation in the mainstem and hilar bronchi, but not the trachea. These data suggest that airway edema could contribute to the chest symptoms caused by inhaled endotoxin.