It has been known for a long time that mental impairment may occur in elderly patients following surgery and this has been attributed to anaesthesia. However, until recently, little systematic research has been carried out in this area. One problem has been that many factors apart from anaesthesia influence the mental function of elderly surgical patients. This paper describes a study in progress which is attempting to isolate the effects of anaesthesia per se on mental function, and identify which aspect of anaesthesia may be responsible. Results from the study indicate that advanced age itself does not predispose to marked postoperative impairment of reaction time, but they do support the idea that patients with some pre-operative mental impairment may be vulnerable to effects of anaesthesia. Thus it appears that the biological age of the brain rather than its chronological age may influence vulnerability to anaesthesia.