A sensitive measure of subtle CNS dysfunction, the continuous reaction time (CRT), was used in 19 patients to quantify the immediate effects of open-heart surgery and extracorporeal circulation (ECC) on the CNS. The control group comprised 17 patients undergoing thoracic surgical procedures without ECC. The reaction time was significantly prolonged after surgery both in the ECC patients and in the controls. CRT was unrelated to the duration of anaesthesia in both groups, but in the ECC group deterioration of CRT showed positive correlation with (a) the duration of ECC, (b) the duration of perfusion pressure below 50 mmHg during ECC and (c) the PCO₂ during ECC. These three factors may be of pathogenetic significance in the development of CNS dysfunction following open-heart surgery. The acute changes in reaction time resolved within a week of surgery, but on questioning two months later half of the ECC patients reported intellectual disturbance, primarily impairment of memory and lability of mood.