To determine whether subjects with mild asthma or seasonal rhinitis have greater bronchomotor responses to sulfur dioxide (SO2) than normal subjects, we undertook a study in 7 asthmatic, 7 atopic, and 7 normal subjects, 23 to 37 yr of age. We measured the change in specific airways resistance (SRaw) provoked by 10 min of breathing 1, 3, and 5 ppm of SO2 delivered by mouthpiece on separate days at least 48 h apart. To assess the significance of parasympathetic pathways in the bronchomotor responses to SO2, we also measured the change in SRaw provoked by 5 ppm of SO2 in 17 of the subjects after they inhaled atropine sulfate aerosol (0.1 mg/kg). We found that in the asthmatic subjects, SRaw increased only at 5 ppm. In the asthmatic group, SRaw increased more than in either of the other groups at 5 ppm (p < 0.005), and was sometimes associated with marked dyspnea requiring bronchodilator therapy. The increases in SRaw produced by inhalation of SO2 were prevented by treatment with atropine in asthmatic and nonasthmatic subjects, which suggested the involvement of parasympathetic pathways. These results indicated that subjects with mild asthma develop bronchoconstriction after exposure to concentrations of SO2 well below currently accepted standards for occupational exposure, and that SO2-induced bronchoconstriction is mediated by parasympathetic pathways.