We measured the influence of changing the end-expiratory lung volume (EELV) on collateral resistance (Rcoll) in 6 normal volunteers (5 male, 1 female, aged 23-45 years). With subjects lying supine in an iron lung, a 5.3 mm fibroscope was introduced under local anesthesia and wedged into a sub-segmental bronchus of the right lower lobe. A truncated 5f Swan-Ganz catheter was tightly fitted into the suction port of the fibroscope. One lumen of the catheter served to deliver a constant flow (200 ml/min) of 5% CO2 in air. The pressure in the wedged segment (Ps) was measured by the other channel. Airway pressure (used as alveolar pressure (Palv)) was determined with a low bias flow catheter taped to the side of the bronchoscope, its tip positioned 20 cm from the end of the bronchoscope. Rcoll was obtained by the formula: Rcoll = (Ps-Palv)/flow. EELV was passively changed by applying a constant extrathoracic pressure in the iron lung at 5 cm H2O steps from 0 to +15 and from 0 to -20 cm H2O. The changes obtained in Rcoll (% baseline, mean +/- SEM) at each extrathoracic pressure (ETP) were: -5 cm H2O, 73 +/- 4%; -10, 56 +/- 7%; -15, 56 +/- 9%; -20, 35 +/- 10%; +5, 118 +/- 6%; +10, 129 +/- 7%; +15, 118 +/- 11%. Rcoll progressively decreased with increasing EELV (negative ETP) and increased slightly as EELV was reduced (positive ETP). The relationship between the log Rcoll and delta EELV was linear; the slope of this line was 9.8%.