The duration of impairment of mental functioning after anaesthesia was studied in 55 patients undergoing hernia repair who were divided into three groups in which the method of induction of anaesthesia (intravenous or inhalational) and ventilation (spontaneous or controlled) was varied. Performance in a five minute serial reaction time test and subjective estimates of coordination were assessed four times a day for two complete postoperative days and were compared with those in a control group of orthopaedic patients in hospital. After considerable impairment initially, reaction times in all groups gradually returned towards control values, but in patients breathing spontaneously during anaesthesia impairment recurred during the second postoperative day. These results suggest that such patients should be advised not to undertake hazardous tasks such as driving a car for at least 48 hours after a general anaesthetic. Discrepancies between subjective and objective assessments of impairment also suggest that patients should not rely on their own assessments of fitness to drive.