The effects of anaesthesia with sufentanil 15 μg kg−1, air and oxygen on the electroencephalograph were studied in 12 patients before and after cardiopulmonary bypass. The e.e.g. responses were characterized by high voltage, slow delta waves. Although the mean power in the delta band declined with time, the contribution of delta power to total power in the frequency range 0.5–40 Hz remained constant until the onset of cardiopulmonary bypass. Sharp waves of uncertain neurophysiological significance were seen. They were not associated with clinical signs of epileptic activity. The use of the e.e.g. response as a monitor of “depth of anaesthesia” during high-dose opioid anaesthesia may become possible.