Significant concentration responses were observed in FVC1 FEV1, FEF 25-75, SGaw, IC, and TLC in 20 healthy, nonsmoking volunteers exposed randomly to 0.00, 0.10, 0.15, 0.20, and 0.25 ppm O3. In addition, significant response changes for FVC1 FEV1, FEF25-75 were shown with time over the 2-h exposure. Intermittent, heavy exercise (VE1 68L/min) lasting 14 min was employed every 30 min during exposure. Inspection of the concentration and time response curves suggests that the threshold for the group response is at or below 0.15 ppm O3. Six subjects experienced decreases > 5% in SGaw at 0.15 ppm. The concentration is only slightly higher than the 1-h O3 National Ambient Air Quality Standard. A dose-related response was also seen for cough, nose and throat irritation, and chest discomfort. The work load, length of exposure, and individual sensitivity must be considered for establishing a safe O3 exposure level.