Twenty-three asthmatic volunteers 19 to 31 yr of age were exposed to 0 (control), 0.2, 0.4, and 0.6 ppm sulfur dioxide (SO2) in random order at 1-wk intervals. Exposures took place in a controlled-environment chamber at 23 degrees C and 85% relative humidity; they included 5 min heavy exercise (mean minute volume, 48 L) plus time for postexercise physiologic testing. Body plethysmography (preexposure and end-exposure), spirometry (end-exposure only), and symptom questionnaires (covering the exposure period and the following week) all showed highly significant trends toward increased response with increasing SO2 concentration. Pairwise statistical comparisons showed substantial, highly significant, changes at 0.6 ppm, relative to control. Fewer and smaller significant changes were found at 0.4 ppm. At 0.2 ppm, no significant physiologic changes were found, but increases in symptoms during exposure were possibly significant. Symptom reports 1 day and 1 wk postexposure showed no significant variation related to SO2 level, i.e., exposure-related symptoms apparently reversed in less than 1 day.