The mechanics of collateral ventilation in the right middle lobe of 5 young, normal volunteers were studied using a bronchoscopic wedge technique, and the results were compared with those from an upper lobe of the same subjects. At functional residual capacity, the resistance to collateral ventilation in the right middle lobe was 4,042 +/- 559 cm H2O per liter per sec (mean +/- SE), whereas that of the upper lobes was 799 +/- 168 cm H2O per liter per sec. The time constant for collateral ventilation could not be measured in the right middle lobe of any of the subjects because it exceeded their breath-holding times of 6 to 16 sec. The time constant for collateral ventilation in the upper lobes was 4.5 +/- 1.0 sec. We concluded that collateral ventilation in the right middle lobe of young normal subjects is characterized by a high resistance and a long-time constant relative to the upper lobes. This is probably explained by a greater ratio of pleural surface to nonpleural surface in the right middle lobe as compared to the upper lobes. We suggest that ineffective collateral ventilation is a major factor in the pathophysiology of the Middle Lobe Syndrome.