The EEG changes produced in humans by fentanyl 30-70 .mu.g/kg during cardiac surgery were defined. Awareness in the patients was assessed. Patients (39) were studied; oral lorazepam or i.m. morphine was used as premedication. Anesthesia was induced with i.v. injection of fentanyl over 2 min and the patients were ventilated with air/O2 (24 patients) or N2O/O2 (15 patients). The EEG 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 done on representative EEG. The EEG effects of fentanyl are consistent and are characterized by high-voltage slow delta waves. N2O had no effect on the EEG responses to fentanyl. Computer analysis confirmed the visual interpretation. There was no incidence of awareness. Fentanyl after premedication is a suitable drug for providing unconsciousness, analgesia and amnesia during cardiac surgery.