Recent data from human studies suggest that the current 1 hr National Ambient Air Quality Standard (NAAQS) for O3 may not be appropriate for exposures of several hours. Animal studies are being used to further investigate this issue. A polynomial model has been developed to depict lung injury from the interaction of O3 concentration (C) and exposure duration (T). The model was derived from lung fluid protein values in rats exposed in a matrix design to 0.1 to 0.8 ppm O3 for 2, 4, or 8 hrs. Airway dysfunction was correlated only at the highest CxT products. Rats, exposed to 0.5 or 0.8 ppm O3 for 2 or 7 hours with intermittent 8% CO2 to augment ventilation, were evaluated with static and dynamic lung function tests. Protein leakage into the airspace was also assessed. Although, the impact of T on O3 toxicity appeared to be C-dependent, loss of function was not necessarily linear. Collectively, studies provide a preliminary basis for the evaluation of duration of exposure on the pulmonary response to O3.