Young male volunteers with mild asthma were exposed for 75 min with natural breathing to 0.0, 0.25, 0.5, and 1.0 ppm SO2. Each exposure included three 10-min periods of treadmill exercise with associated minute ventilation. Specific airway resistance (sRaw) was not significantly increased after exercise in 0.25 ppm SO2. In 0.5 and 1.0 ppm SO2, sRaw was increased 2-fold and 3-fold above preexposure levels respectively. There was a broad range of responses to exercise and SO2. The increases in sRaw after the second and third exercises were significantly less than after the first exercise. Respiratory impedance measured by forced random noise suggests that the induced bronchoconstriction was primarily associated with peripheral airways. The results confirm that mild asthmatics as a group have significant bronchoconstriction in response to short-term moderate exercise with natural breathing in 1.0 and 0.5 ppm SO2 but not in 0.25 ppm SO2. In addition, there is less induced bronchoconstriction when exercise is repeated at short intervals during exposure to SO2.