The postoperative course of mental performance during the first week and at 3 months after operation was studied in 30 patients above the age of 60, undergoing total hip replacement arthroplasty. The patients were randomly allocated to receive either general anesthesia, epidural analgesia, or general anesthesia plus epidural analgesia. The surgically induced increase in plasma cortisol and glucose was inhibited in the two groups receiving epidural analgesia. Mental performance was studied with psychological methods. An equal degree of postoperative impairment of mental performance of 3-4 days' duration was found in all groups. Three months after surgery, mental function had improved slightly and to the same extent in all groups. We conclude that the after surgery, mental function had improved slightly and to the same extent in all groups. We conclude that the transient mental impairment occurring within the first postoperative week is caused by factors other than general anesthetic agents and the endocrine-metabolic response to surgery.