Studies in both humans and rats have indicated that certain pulmonary responses induced by exposure to an acute provocative concentration of ozone (O3) will eventually attenuate if the exposure is repeated on a daily basis. This phenomenon is commonly referred to as O3 adaptation. Whether or not a "state" of adaptation develops due to long-term low level O3 exposure is unknown. Two human studies have reported adaptation in subjects living in Los Angeles during periods when ambient O3 concentrations have been relatively high. At present, however, we are not aware of comparable information from rats. This study assessed O3 adaptation in rats following chronic (12 or 18 months) exposure and after a 4-month recovery period. A chronic exposure pattern, similar to that found in an urban area during the summer (0.06 ppm O3 for 13 hr/day, 7 days/week; Monday-Friday, peak to 0.25 ppm O3, over 9 hr), was used. To assess whether adaptation had occurred and/or persisted, awake rats were challenged with high provocative concentrations of O3 for up to 2 hr. During a challenge, rats were monitored for typical O3-induced alterations in spontaneous breathing parameters (e.g., increase in breathing frequency and decrease in tidal volume). Adaptation was defined as attenuation of breathing response during the challenge in rats chronically exposed to O3 as compared to that in "control" rats (chronically exposed to air). Adaptation was found in the rats within 8 hr following the chronic O3 exposure but not after the 4-month recovery period. Spontaneous breathing parameters that were significantly attenuated in the chronically exposed rats were breathing frequency, tidal volume, inspiratory and expiratory times, and maximum expiratory flow. We conclude that rats demonstrated adaptation to O3 after long-term exposure to an urban-type O3 profile and that the adaptation was not seen 4 months postexposure. These results suggest that exposure to environmental O3 in Los Angeles air may have been responsible for the adaptation found in residential subjects.