1. In spontaneously breathing dogs (n = 8), maintained in the upright position, bronchial luminal diameter was measured from tantalum bronchograms. Changes in total intrathoracic lung volume were measured from radiographs taken at functional residual capacity (FRCR).
2. With cooling of the cervical vagi to 0–4°C mean bronchial diameter increased to 117 ±15 (sd)% of baseline diameter and FRCR increased to 113 ± 16 of baseline volumes respectively. There was a significant correlation between changes in FRCR and bronchial diameter.
3. After the vagi were rewarmed, pulmonary oedema was induced by rapid intravenous infusion of Hartmann's solution. Bronchi narrowed to a mean of 86 ± 9% of baseline calibre and FRCR decreased (mean = 95 ± 15%).
4. With vagotomy bronchial diameter increased (mean = 102 ± 12% of baseline diameter) but FRCR did not change significantly.
5. Excluding one dog with gross oedema, changes in bronchial diameter due to oedema correlated positively with changes in FRCR; after vagotomy the relationship between diameter and FRCR was similar to that seen after vagal cooling in the baseline state, though individual values were lower.
6. The shift in the diameter-FRCR relationship with vagotomy demonstrates that the direct bronchoconstrictor effect of the vagus is approximately doubled in the presence of pulmonary oedema.
7. Bronchial and peribronchial oedema does not appear to directly narrow the bronchial lumen.