We sought to determine whether 0.25 ppm sulfur dioxide in filtered air causes bronchoconstriction when inhaled by freely breathing, heavily exercising, asthmatic subjects. Nineteen asthmatic volunteers exercised at 750 kilogram meters/min for 5 min in an exposure chamber that contained filtered air at ambient temperature and humidity or, on another day, filtered air plus 0.25 ppm sulfur dioxide. The order of exposure to sulfur dioxide and to filtered air alone was randomized, and the experiments were double-blinded. Specific airway resistance, measured by constant-volume, whole-body plethysmography, increased from 6.38 +/- 2.07 cm H2O X s (mean +/- SD) before exercise to 11.32 +/- 8.97 after exercise on days when subjects breathed filtered air alone and from 5.70 +/- 1.93 to 13.33 +/- 7.54 on days when subjects breathed 0.25 ppm sulfur dioxide in filtered air. The increase in specific airway resistance on days when subjects breathed 0.25 ppm sulfur dioxide was only slightly greater than on days when they breathed filtered air, but the difference was significant. To determine whether 0.25 ppm sulfur dioxide causes greater bronchoconstriction in asthmatic subjects exercising more vigorously, 9 subjects then repeated the experiment exercising at 1,000 instead of 750 kilogram meters/min. Specific airway resistance increased from 6.71 +/- 2.25 to 13.59 +/- 7.57 on days when subjects breathed filtered air alone and from 5.23 +/- 1.23 to 12.54 +/- 6.17 on days they breathed 0.25 ppm sulfur dioxide in filtered air. The increase in specific airway resistance on the 2 days was not significantly different.(ABSTRACT TRUNCATED AT 250 WORDS)