During anaesthesia and during the management of respiratory failure by intermittent positive pressure ventilation, incipient hypoxaemia is common owing to the frequent occurrence of an increased physiological shunt. This hypoxaemia may be offset by raising the inspired oxygen tension and by hyperventilation. The former may produce alveolar collapse over short periods, and pulmonary oxygen toxicity if sustained. The latter can be shown to exaggerate hypoxaemia under certain circumstances.