Bronchial pressure, esophageal pressure, and flow at the mouth were measured during cinebronchography in three normal subjects, three patients with emphysema, and one subject with intermittent bronchospasm. Each cineframe exposure generated an electrical signal which was recorded simultaneously with pressures and flow. Since the bronchial catheter had a 1-cm radio-opaque tip, bronchial diameters were obtained simultaneously with bronchial pressure, esophageal pressure, and flow. By this technique the pressure drop down the bronchial tree during forced expiration was measured. In normals, although greater than 50% of the total pressure was required to produce flow from the right lower lobe bronchus to the mouth, the pressure drop down its length alone was only 2–15% of the total, whereas in emphysema it was 35–85% and was associated with collapse of this bronchus. The narrowing of large bronchi, in particular those with posterior membranous sheaths, is thus a factor in limiting expiratory flow in normal subjects. In the three patients with emphysema the collapsing right lower lobe bronchus impeded emptying of the lower lobe during forced expiration, apparently due to an increase in the compliance of this bronchus. Submitted on November 15, 1962