Episodes occasionally occur when ambient ozone (03) levels remain at or near 0.12 ppm for more than 6 h. The hypothesis that prolonged exposure to 0.12 ppm 03 would result in progressively larger changes in respiratory function and symptoms over time was tested. Ten nonsmoking males (18-35 yr) were exposed once to clear air (CA) and once to 0.12 pp, 03 for 6.75 h. Exposures consisted of six 50-min exercise periods, each followed by 10-min rest and measurement; a 45-min lunch period followed the third exercise period. Exercise ventilation averaged approximately 40 1/min. Forced expiratory and inspiratory spirometry and respiratory symptoms were measured prior to exposure and after each exercise. Increases in the symptom ratings of cough and pain on deep inspiration were observed with 03 exposure but not with CA. Airway reactivity to methacholine was approximately doubled following 03 exposure. Spirometry results indicate that prolonged exposure to 0.12 ppm 03 results in a marked increase in non-specific airway reactivity and progressive changes in respiratory function.