During bronchoconstriction elastic after-loads arise due to distortion of lung parenchyma by the narrowing airway. In the present study, the functional effect of parenchymal elastic after-load on airway narrowing was determined.Airway narrowing was measured in vivo over a range of transpulmonary pressures and compared with in vitro narrowing measured at corresponding transmural pressures. Bronchi were generation 10 with internal diameters of similar to 4 mm. In vivo luminal narrowing was measured by videobronchoscopy in anaesthetised and ventilated pigs. In vitro luminal narrowing was measured by videoendoscopy in isolated bronchial segments. Airways were activated by maximum vagal nerve stimulation and maximum electrical field stimulation in vivo and in vitro, respectively.At 5 cmH(2)O, stimulation produced a 35.9 +/- 3.2% (n=6) and a 36.5 +/- 2.4% (n-11) decrease in lumen diameter in vivo and in vitro, respectively. At 30 cmH(2)O, luminal narrowing fell to 23.7 +/- 2.0% in vivo and 23.4 +/- 2.5% in vitro. There was no difference between luminal narrowing in vivo and in vitro at any pressure.In conclusion, these findings suggest that in mid-sized, cartilaginous bronchi, parenchymal elastic after-loads do not restrict airway narrowing.