To investigate the potential for near-ambient levels of nitrogen dioxide (NO2) to induce functional lung damage, groups of rats were exposed to air or a simulated urban profile of NO2 (0.5 ppm, 1.5 ppm peak) for 1, 3, 13, 52, or 78 weeks. The dynamic, static, and diffusional characteristics of the lung were evaluated postexposure in anesthetized rats. Furthermore, for the 13-, 52-, and 78-week groups, additional animals were tested after a 6-, 26-, or 17-week period in filtered air, respectively. Breathing patterns and mechanics were also assessed postexposure in a parallel group of similarly exposed unanesthetized rats during filtered air and 4 and 8% carbon dioxide (CO2) challenge. No significant NO2 effects were found for the nitrogen washout, compliance, lung volumes, diffusion capacity of carbon monoxide, or upstream airway resistance. However, at 78 weeks, a significant reduction in Delta FEF25, an estimate of convexity in the later portion of the forced expiratory flow volume curve, was observed. Decreases in Delta FEF25 suggest premature small airway closure. In the unanesthetized rat, frequency of breathing was decreased and tidal volume, expiratory resistance, and inspiratory and expiratory times were generally increased. For several of these variables, the largest response also occurred at 78-weeks and seemed to be exacerbated by CO2 challenge. Data from both unanesthetized and anesthetized pulmonary function tests suggest the presence of small airway obstruction or premature closing.