The purpose of this study was to investigate the state of wakefulness during the induction of anesthesia with high-dose fentanyl using the isolated forearm technique. Ten patients scheduled for elective cardiovascular surgery were premedicated with morphine (0.15 mg/kg) and scoploamine (0.3-0.4 mg) intramuscularly one hour before induction. The induction of anesthesia was performed by intravenous administration of 100 micro g/kg of fentanyl in 15 min or over. The pneumatic tourniquet applied on the left upper arm was inflated to 220-240 mmHg after 10 micro g/kg of fentanyl was given and then pancuronium was administered. Verbal commands were given to the patient after 25, 50, 75 and 100 micro g/kg of fentanyl was administered. Eight patients out of 10 responded to the verbal commands after administration of 25 micro g/kg of fentanyl. Six patients also responded after administration of 100 micro g/kg of fentanyl and diazepam 5 mg was given to prevent tachycardia and rigidity during endotracheal intubation. Muscle rigidity and tachycardia were noticed in three and four patients respectively. These complications disappeared by diazepam administration. It was noted that wakefulness frequently occurred during the induction by high-dose fentanyl and oxygen anesthesia. To prevent such wakefulness therefore, it is necessary to use anesthetic supplements which do not cause cardiovascular depression.