The effects of different anaesthestics on cognitive functions in elderly, air-breathing patients undergoing prostatectomy were studied and the results compared with those of patients who received oxygen additional to that in air The patients received extradural anaesthesia, air and 3–4% ether, air and 0.5% trichloro-ethylene or nitrous oxide, oxygen and intermittent i.v morphine or pethidine. The first three groups received no additional oxygen during or after operation; the fourth received oxygen for 12 h after operation. There were 15 patients in each group. The tests used were developed as appropriate for illiterate as well as educated patients. No deterioration of cognitive function was found in any group