Awake Fischer-344 rats were exposed to O3 and response was evaluated before, during and after the exposure using a protocol similar to that used in many human studies. To enhance test sensitivity, CO2 was periodically added to the O3-air exposure mixture. Fifteen min. alternate periods of filtered air or 2, 4, 6, or 8% CO2 were used to stimulate ventilation and to increase pollutant uptake, much like exercise in controlled human exposure studies. Ventilation and breathing mechanics were evaluated at 0.0, 0.12, 0.25, 0.5 and 1.0 ppm O3, while ventilation and blood pressure, blood gases and the electrocardiogram were evaluated at 0.0 and 1.0 ppm O3. Control experiments provided data on the effect of ozone exposure to 0.5 ppm O3 without added CO2 challenge. Results indicated that CO2 enhanced the detection of O3 effects at concentrations lower than 0.5 ppm O3. As O3 concentration increased, the magnitude of ventilatory response (decrease in tidal volume and increase in frequency of breathing) was greater and the onset of effect was progressively sooner. Breathing mechanics were not affected by O3 exposure except for an increase in expiratory flow rates near the end of the tidal breath. (Copyright (c) 1990 by John Wiley and Sons, Ltd.)