Thirty healthy patients subjected to cholecystectomy or operation for duodenal ulcer were allocated randomly for postoperative analgesic treatment with morphine i.m. or epidurally. Morphine was given only at the request of the patients and only as much was given as was needed to obtain satisfactory pain relief. Patients in the epidural group were given morphine exclusively by epidural injection. In the epidural group a lower incidence of radiological changes in the lungs was found postoperatively - 21% as against 67%. Compared with the i.m. group, there was a higher arterial oxygen tension and a slower increase in alveolar-arterial oxygen difference. It is concluded that epidural morphine analgesia reduces the degree of postoperative lung dysfunction compared with conventional i.m. morphine treatment.