We studied 10 nonsmoking young adults before and after inducing asthmatic attacks by treadmill exercise. We used body plethysmography, flow-volume curves with air and a mixture of 80% helium and 20% oxygen, pressure-volume diagrams, and arterial blood gas analyses to characterize the effects of exercise and acute inhalation of 6% CO2. Even when exercise produced no change in arterial CO2 tension, inhalation of 6% CO2 relieved obstruction to airflow. It also altered the volume-pressure ralationship of the lungs so that total lung capacity was reduced within minutes, and elastic recoil was increased at fixed lung volume. A large increase in density dependence of airflow was seen in some cases, suggesting relief of obstruction in peripheral airways. Atropine sulfate did not prevent obstruction after exercise and did not prevent relief during CO2 inhalation. We concluded that CO2 inhalation can relax both central and peripheral airways in young asthmatic adults, both at rest and after exercise, and that both total lung capacity and density dependence of airflow can change acutely in these subjects.