Prediction equations developed from previous ozone (O3) exposure studies suggested that athletes exercising at near competitive intensities would be subject to alteration of pulmonary function during exposure to relatively low concentrations of O3. Accordingly we exercised seven trained athletes for 1 h at 75% of maximal O2 consumption in both room air and a 0.21 ppm O3 environment. Pulmonary function tests, including forced expiratory maneuvers and maximum voluntary ventilation (MVV), were performed prior to and immediately following the 1-h test. Significant decreases in forced vital capacity (FVC, -7%), forced expired volume in 1.0 s (-15%), forced expiratory flow over the midhalf of FVC (-18%), and MVV (-17%) were recorded following O3 exposure. The magnitudes of these changes are similar to those observed in subjects performing moderate intermittent exercise for 2 h in a 0.24 ppm O3 environment. Symptoms reported following O3 exposure included laryngeal and/or tracheal irritation and soreness and chest tightness on taking a deep breath. The observed alterations in lung functions in these subjects indicate that individuals performing heavy continuous exercise are more likely to be affected by lower O3 levels.