Airway resistance, FEV1.0 and lung volume were measured by body plethysmography before and after voluntary hyperventilation. In normal subjects, resistance increased to 130--140% of the initial value measured before hyperventilation. The same increase was observed in silicosis patients without chronic obstructive bronchitis. Asthmatic patients in an asymptomatic phase showed a rise in airway resistance to an average of 255% of the nearly normal initial values, and also a reduction in FEV1.0. In normal subjects and asthmatic patients, the administration of bronchodilators inhibits the rise in airway resistance induced by hyperventilation. The hyperventilation test can be used to identify increased susceptibility to bronchoconstriction.