The purpose of the study was to determine the shortest duration of exposure to 1.0 ppm sulfur dioxide (SO2) sufficient to induce bronchoconstriction significantly greater than that observed with exposure to clean air (CA) in exercising SO2 sensitive asthmatics. Asymptomatic, nonmedicated, male asthmatics (n=12) with airway hyperresponsiveness to both methacholine and SO2 were exposed in a chamber (20 degree C, 40% relative humidity) for 0.0, 0.5, 1.0, 2.0 and 5.0 min to both CA and 1.0 ppm SO2 on separate days (10 exposures). Just prior to each exposure, subjects walked on a treadmill in CA for 5 min at a predetermined speed/elevation to elicit a target ventilation of about 40 L/min. i.e., a brisk pace up a slight incline. After this walk, subjects rapidly entered adjoining exposure chamber containing either CA or SO2 and immediately walked at the same speed/elevation for the specified exposure duration. Subjects then rapidly exited the chamber. Postexposure SRaw and symptom ratings increased with increased exposure duration in SO2; pstexposure SRaw also was increased with increased exposure duration in CA but to a lesser extent. After adjusting for the CA response, significantly greater SO2 induced bronchoconstriction was observed for the 2.0 and 5.0 min exposures as indicated by substantially greater increases in SRaw and substantially higher ratings of respiratory symptoms. The authors conclude that with the above exposure conditions, on average, SO2 sensitivie asthamtics exhibit significant brochoconstriction at exposure durations of 2.0 min or more.