Nineteen volunteers breathed 100% oxygen for 10 min and then for a further 5 min at a restricted lung volume. Residual volume (RV) was shown to be significantly decreased by 120 ml when compared with control values breathing air. Functional residual capacity (FRC) also was decreased significantly by this manoeuvre. Other measurements indicated that the volume of FRC minus the closing capacity (CC) (FRC-CC) was not of importance in the development of the reduction in RV. It is postulated that such decreases in alveolar volume may contribute to postoperative hypoxia and morbidity.