We measured the diameter and vertical displacement of the laryngeal and oropharyngeal airways in 8 seated asymptomatic asthmatic subjects (31 to 60 yr of age) before and after bronchoconstriction (BC) induced by histamine phosphate aerosol. An image of the airways was obtained in 2 planes using an X-ray fluoroscope attached to a videotape recorder, onto which signals of flow and volume at the mouth were recorded simultaneously. We calculated real dimensions from the video image using size markers included in the picture. During BC, when FEV1 fell to 35 +/- 4% (mean +/- SE) of control, the lateral diameter (DLAT) of the larynx at mid-expiration narrowed over approximately 10 mm vertical height above the glottis. Lateral diameter of the glottis during mid-expiration fell from 6.5 +/- 1.5 mm by 46 +/- 12% (p less than 0.05), but that during inspiration was unchanged. In the 3 subjects with the greatest glottic narrowing, anteroposterior diameter (DAP) and DLAT of the larynx at the level of the corniculate cartilages narrowed by 28 and 27%, respectively. There was inspiratory dilatation of the oropharynx during BC. The DAP during inspiration at the level of the hyoid increased from 15.0 +/- 1.5 to 17.5 +/- 1.5 mm (p less than 0.05), while that at the level of the third cervical vertebra (C3) increased from 12.5 +/- 1.5 to 17.0 +/- 2.0 mm (p less than 0.05). The diameters during mid-expiration were unchanged so that the difference between inspiration and mid-expiration increased at both levels. Higher in the oropharynx (20 mm above C3), expiratory DAP decreased in 6 of 8 subjects.(ABSTRACT TRUNCATED AT 250 WORDS)