We measured the cross-sectional area of the glottis in 12 asymptomatic asthmatic subjects before and after bronchoconstriction (BC) induced by histamine or ultrasonically nebulized water aerosol. The glottis was visualized using a fiberoptic bronchoscope attached to a video camera and tape recorder onto which flow and volume were simultaneously recorded. During induced asthma, measurements of forced expiratory flow fell to 36 +/- 3% (mean +/- SE) of control. Glottic area at mid-expiration (Age) fell from 75 +/- 14 mm2 by 45 +/- 8% (p less than 0.005). In some subjects, expiratory constriction of the supraglottic portion of the larynx and pharynx was also noted. Five subjects showed mild constriction (13 to 32%) of the glottis during inspiration. The administration of 10 cm H2O of continuous positive airway pressure (CPAP) during the induced asthma temporarily abolished expiratory constriction of the glottis and supraglottic structures; Age increased from 45 +/- 15 to 79 +/- 11 mm2 (p less than 0.01). The constriction returned when CPAP was stopped. After reversal of BC by salbutamol Age returned to values similar to those before BC. Although the mechanism for the expiratory glottic constriction is not known, it may be related to the different pattern of respiratory muscle activity seen in asthma. We speculate that the glottic constriction may contribute to hyperinflation by slowing expiratory flow and/or allowing a reduction in the persistent inspiratory muscle activity during expiration.