A group of male amateur cyclists was studied in the summer of 1991 in the east of the Netherlands. Lung function was measured before and after training sessions or competitive races on a number of occasions. Continuous heart rate monitors were employed to document exercise levels. Heart rate averaged 161 beats/min during training, and 176 beats/min during races. Exercise duration averaged 75 min. Ozone concentrations during exercise were obtained from the nearest stations of the National Monitoring Network. The difference between pre- and postexercise lung function values was related to these ozone concentrations. Ozone concentrations were low on most occasions with an average of 87 micrograms/m3 and a maximum of 195 micrograms/m3. The difference between pre- and postexercise lung function was found to be negatively related to the ozone concentration during exercise. When all observations obtained at ozone concentrations higher than 120 micrograms/m3 were removed from the analysis, the relationship with ozone was still significant. The data also suggested that effects of ozone on lung function were stronger in midsummer than in the late summer. The difference between pre- and postexercise acute symptoms was positively related to ozone, for shortness of breath, chest tightness, and wheeze. Cough and eye irritation were not related to ozone. These results indicate that in healthy young men vigorously exercising outdoors, ozone is related to lung function changes and acute respiratory symptom changes at low levels of exposure.