The purpose of this study was to define the EEG changes produced in humans by fentanyl 30-70 microgram/kg during cardiac surgery. The authors have also assessed awareness in the patients. Thirty-nine patients were studied; oral lorazepam or intramuscular morphine was used as premedication. Anesthesia was induced with intravenous injection of fentanyl over 2 min and the patients were ventilated with either air/O2 (24 patients) or N2O/O2 (15 patients). The EEGs recorded until the start of cardiopulmonary bypass were visually analyzed, classified into EEG stage, and plotted graphically as narcograms. Computerized 3-dimensional power spectral analysis and wide band spectral analysis were carried out on representative EEGs. The EEG effects of fentanyl are consistent and are characterized by high-voltage slow delta waves. Nitrous oxide had no effect on the EEG responses to fentanyl. Computer analysis confirmed the visual interpretation. There was no incidence of awareness. The authors conclude from this study that fentanyl after premedication is a suitable drug for providing unconsciousness, analgesia, and amnesia during cardiac surgery.