To investigate the mechanism of exercise-induced bronchospasm, we measured specific airway conductance before and after exercise in 7 healthy normals, 12 asthmatics with intact carotid bodies, and 5 asthmatics who had had bilateral carotid body resection. The subjects breathed either air or oxygen (randomly assigned) during cycle ergometer exercise. Post-exercise bronchodilation was the usual pattern in normals, whereas post-exercise bronchospasm occurred in all asthmatics who breathed air during exercise. Oxygen breathing during exercise markedly attenuated the post-exercise bronchospasm in those asthmatics with intact carotid bodies, but had no significant effect in those without effect in those without carotid bodies. The attenuation of the bronchospasm with oxygen occurred with either incremental or constant load exercise of high intensity. The degree of attenuation did not correlate significantly with changes in end-tidal PCO2, maximum work rate, maximum exercise ventilation, or maximum heart rate. These studies indicate that oxygen attenuates exercise-induced bronchospasm in asthmatics through its action on the carotid bodies.