Changes in respiratory function have been suggested for children exposed to less than 0.12 ppm ozone (O3) while engaged in normal activities. Because the results of these studies have been confounded by other variables such as temperature or the presence of other pollutants or have been questioned as to the adequacy of exposure measurements, the authors determined the acute response of children exposed to 0.12 ppm O3 in a controlled chamber environment. Twenty-three white males ages 8-11 years were exposed once to clean air and once to 0.12 ppm O3 in random order. Exposures were for 2.5 hours and included 2 hours of intermittent heavy exercise. Measures of FEV1 and the symptom cough were determined prior to and after each exposure. A significant decline in FEV1 was found after the O3 exposure compared to the air exposure and appeared to persist for 16-20 hours.