Histologic studies of large airways suggest that outer airway wall area increases during bronchoconstriction, which could influence the relationship between external diameter and lumen flow. Using isolated bronchi from pigs we simultaneously recorded external diameter using sonomicrometry, and lumen flow of liquid using an electromagnetic flowmeter to determine the relationship between the two. External diameter fell from 4.13+/-0.19 to 3.65+/-0.19 mm (11+/-3%, n = 5) during maximal electrical field stimulation (EFS), and flow decreased by 67+/-9%. External diameter plotted against lumen flow showed hysteresis between contraction and relaxation. External narrowing ceased towards the end of a stimulation period, but flow continued to decrease. This differed from predictions based on an assumption that the wall area does not change during contraction. Histologic sections of bronchi fixed after acetylcholine (ACh) challenge showed an increase in total wall area. These results illustrate dynamic wall movement during bronchoconstriction induced by EFS or acetylcholine.