A case of haemodynamic decompensation is reported in a patient with a sick-sinus syndrome on a demand cardiac pacemaker, under-going surgery under general anaesthesia. While systemic arterial blood pressure showed adequate stability under the patient's own sinus drive, it consistently fell by 15 to 20 p. cent when artificial pacing was established. The possible mechanisms are discussed. The drop in arterial blood pressure with artificial pacing is probably due to the loss of the atrial contribution in maintaining adequate cardiac output. In the conscious patient, the resulting decrease in cardiac output is compensated by an increase in the peripheral resistance. Under general anaesthesia, this compensatory mechanism is lost, resulting in a drop in cardiac output and systemic arterial blood pressure. It needs to be emphasized therefore, that the patient who has a permanent pacemaker has significant under-lying cardiovascular disease and, that, continuous monitoring is indicated not only during surgery but also postoperatively until patient has stabilized.