Because minimal data are available regarding the pulmonary effects of ozone (O3) at levels < 0.27 ppm, six groups of healthy young males were exposed for 2.5 h to one of the following O3 concentrations:0.0, 0.12, 0.18, 0.24, 0.30, or 0.40 ppm. Fifteen-minute periods of rest and exercise (65 1/min minute ventilation) were alternated during the first 2 h of exposure. Coughing was observed at all levels of O3 exposure. Small changes in forced-expiratory spirometric variables spirometic variables (forced vital capacity (FVC), forced expiratory volume in 1s, and mean expiratory flow rate between 25 and 75% FVC) were observed at 0.12 and 0.18 ppm O3, and larger changes were found at O3 levels greater than or equal to 0.24 ppm. Changes in tidal volume and respiratory frequency during exercise, specific airway resistance, the presence of pain on deep inspiration, and shortness of breath occurred at O3 levels greater than or equal to 0.24 ppm.