Methods of monitoring airway caliber as an indicator of bronchomuscular tone have been indirect, static, and invasive. A diameter transducer has been developed the static and dynamic characteristics of which indicate that it should overcome some methodological problems. The catheter-mounted spring-loaded braces of the transducer track the motion of the airway wall, which is reflected in the output changes of a linear-variable transformer. The range of the transducer was 5.3-24.3 mm. The amplitude response was flat up to 8 Hz, and the phase difference between the forcing signal and the response of the gauge was 0 degrees up to 8 Hz. The minimum specific compliance was 0.15 cmH2O-1, an order of magnitude larger than the specific compliance of isolated trachea, indicating that the airway was not significantly loaded by the transducer. In vivo studies in ventilated dogs revealed tidal changes in tracheal diameter and marked constriction during apnea. Bilateral vagotomy increased diameter up to 20%, and vagal stimulation decreased diameter as much as 30% below postvagotomy levels. It appears that this transducer is suitable for measuring resting diameter, tidal changes in diameter, and changes in diameter induced by alteration in bronchomotor tone.