In dogs anesthetized with Na barbital, the gases respired collaterally by obstructed long lobules were collected, measured, and analyzed. In lobular obstruction, where collateral respiration is effective, the gas in the obstructed area never shows a CO2 tension above the arterial CO2 level, and shows a correspondingly higher O2 tension. An obstructed lung lobule may breathe through collateral respiratory channels at least 10% as much air as it would normally through direct bronchial channels. The volume-flow of collateral air varies directly and proportionately within certain limits to the depth of respiration (tidal volume) ; except when positive pressure breathing is instituted, this causes the volume flow of collateral air to increase out of proportion to the depth of respiration. Collateral respiration appears to have economic significance in bronchial obstruction, in that it permits an obstructed lung field to carry on a considerable ventilation which may be sufficient, indeed, to oxygenate the venous blood passing through the vessels in the obstructed area.