The effectiveness of collateral ventilation in maintaining alveolar gas tensions in obstructed lung segments was investigated using fiberoptic bronchoscopy to place an occluding catheter-tip balloon in selected lobar and segmental bronchi in supine normal human subjects. Gas tensions from beyond the occlusion were measured with a respiratory mass spectrometer. Collateral ventilation is known to be minimal between lobes; therefore, values measured in obstructed lobes provide a control. No significant difference was found between the partial pressures of oxygen or carbon dioxide measured in obstructed lobes and in obstructed segments. In both cases respiratory gas tensions approached reported values for mixed venous levels. The time taken to attain a steady state of gas composition in the obstructed lung was rapid (approximately 50 s), and it was no different for lobes and segments. In addition, collateral ventilation was assessed by measuring the amount of helium reaching occluded lobes and segments when subjects breathed a mixture of 21% oxygen and 79% helium. The rate of rise in helium concentration was less than 1%/min in both lobes and segments, a figure that may be explained by delivery of helium in recirculated blood rather than by collateral ventilation. We conclude that intersegmental collateral ventilation has a negligible role in the maintenance of alveolar gas tensions in supine normal humans during tidal breathing.