Six patients were studied following general anaesthesia for cholecystectomy or hip replacement. Intravenous morphine was given for postoperative pain relief. Continuous measurements were made of breathing pattern and arterial oxygen saturation for a 12-hour period postoperatively. The effect of breathing either air or 28% oxygen for alternate 2-hour periods was examined. There was no significant effect of oxygen on the number of periods of central apnoea, obstructive apnoea or partial upper airways obstruction. The number of episodes of decreases in oxygen saturation to below 80%, associated with these breathing disturbances, was reduced from 59 to zero by the administration of oxygen. There was a gradual improvement in oxygenation whilst breathing air during the 12-hour postoperative period. The administration of oxygen had a beneficial effect on average arterial oxygen saturation.