Ozone (O3) is a common air pollutant that has been associated with a dose-dependent increased bronchial responsiveness and airway inflammation. Previous investigations have shown increased airway responsiveness to allergens in asthmatics pre-exposed to 0.12 ppm of O3 for 1 h. In the present study, we investigated whether inhalation of relatively low levels of O3 would modify the degree of exercise-induced bronchoconstriction. We studied 15 "never smokers" with mild stable asthma (7 male and 8 female) (mean age [+/- SD] 25.6 +/- 6.8 years) who had exhibited a fall in FEV1 > 15 percent after a standard 6-min treadmill exercise challenge test on the screening day. This was a double-blind, placebo-controlled study. The patients were randomized to receive either O3 or air (placebo) before performing the exercise challenge again. The average highest 1-h daily O3 concentrations in Toronto during O3 days and air days were 0.017 +/- 0.017 and 0.014 +/- 0.005 ppm, respectively. The O3 concentration inside the chamber averaged 0.122 +/- 0.005 ppm on O3 days and 0.002 +/- 0.001 on placebo days. Partial and complete flow volume curves were done before and after this exposure, and also 5, 10, 15, 20, 30, and 60 min postexercise. The percent fall in FEV1 on the O3 chamber day and on the air chamber day was the same (F = 0.67, p = 0.67, NS) as well as the percent fall in V40p (F = 0.91, p = 0.49, NS). A repeated measures analysis of variance to test the effects of exposure on the time course of the airway response after exercise showed no significant difference between the 2 days. There was also no significant difference in maximal percentage fall in FEV1 (25.6 +/- 8.6) or V40p (62.2+18.6) following O3 exposure, and FEV1 (26.8 +/- 9.4)(p = 0.64) or V40p (65.3+4.31)(p = 0.60) following air. Our data indicate that previous exposure at rest to a concentration of O3 that has previously been shown to augment the bronchoconstriction response to allergens did not increase the bronchoconstriction response to subsequent exercise nor did it change the time course of such bronchoconstriction.