Ventilation at maximal voluntary reduction of lung volume caused significant desaturation in some healthy subjects breathing air. Saturation rapidly returned to control levels when normal lung volume was regained. These changes are probably due to reversible airway obstruction. During the inhalation of oxygen, ventilation at maximal voluntary reduction of lung volume caused, in one subject, a reduction of arterial Po2 of 243 mmHg. Normal arterial Po2 was not immediately restored on regaining normal lung volume. Chest radiographs showed extensive atelectasis, which persisted for several hours in an ambulant subject. These changes are probably due to absorption of oxygen from alveoli beyond obstructed airways. Reduction of lung volume may be harmful for patients who are breathing oxygen. Caution is therefore necessary in the use of a subatmospheric pressure phase during artificial ventilation and during suction of the tracheobronchial tree.