This study was designed to determine whether morphine 0.1 mg.kg-1 i.v. given intraoperatively altered the end-tidal concentration of sevoflurane which is associated with eye opening to verbal command. We studied 24 healthy ASA physical status I patients to determine whether morphine, or placebo administered about 60 min before the end of surgery affected recovery from sevoflurane/oxygen anaesthesia. During anaesthesia no other anaesthetics or drugs were given. After surgery, end-tidal sevoflurane concentration was reduced gradually at the rate of less than 0.01%.min-1. The end-tidal concentration at the time patients could respond to verbal command was recorded as MACawake. The MACawake was 0.58 +/- 0.12% (mean +/- SD) for the control group to whom placebo had been administered, and 0.57 +/- 0.11% for morphine group to whom morphine had been administered. In both groups, the MACawake decreased with age, and the ratio to age-adjusted sevoflurane MAC was 0.31 +/- 0.04 (mean +/- SD) for the control group and 0.30 +/- 0.04 for the morphine group. The ratio had no correlation with age. It is concluded that the awakening concentration of sevoflurane during recovery from anaesthesia is not affected by analgesic doses of morphine 0.1 mg.kg-1 i.v. administered intraoperatively.