Intermittent exposures of rats to ozone (O3) of 1 and 0.5 ppm were performed. Flow resistance of the lungs left in situ in the chest cavity opened after the sacrifice was measured at different levels of elastic pressure, and it was shown that daily 3-hr exposures to 1 ppm O3 for 30 consecutive days mainly produced increased resistance of more-central airways, while daily 6-hr exposures to 0.5 ppm O3 for 60 consecutive days mainly increased the resistance of peripheral airways. Morphological changes of the lungs caused by both exposures were in general similar in nature, but differences in secretion were noticed: it was more evident in larger to middle-sized bronchi in the animals intermittently exposed to 1 ppm O3, but more evident in the peripheral airway region in the animals intermittently exposed to 0.5 ppm O3. No change in the pulmonary static volume-pressure curve, indicative of the altered retractive forces of alveoli, was observed in both exposures, although a slight reduction in lung distensibility was suggested.