We have studied the effect of FIO2 of a nitrous oxide-oxygen mixture on the rate of gas uptake from an unventilated lung. Nine anaesthetized dogs were studied, each breathing four nitrous oxide-oxygen mixtures (FIO2 0.3, 0.5, 0.75 and 1.0) in random order. A double-lumen endobronchial tube separated lung ventilation. Both lungs were given the nitrous oxide-oxygen mixture for an equilibration period. Then the right lung was connected to a spirometer containing the same gas, and gas uptake measured. In every dog, gas uptake was faster with an FIO2 of 0.5 or 0.75 than with an FIO2 of 1.0. When breathing a nitrous oxide-oxygen mixture with FIO2 > 0.3, the rate of gas uptake from the unventilated lung was faster than with 100% oxygen.