BACKGROUND: During recovery from halothane-N2O anesthesia, the elimination of nitrous oxide accelerates that of halothane by diluting the alveolar concentration of halothane and increasing the expired volume(""reversed 2nd gas effect""). Also the higher the inspired concentration of nitrous oxide, the more rapid is the alveolar rate of fall(""reversed concentration effect""). This present study was performed to determine that the nitrous oxide elimination accelerates the elimination of enflurane both by ""reversed 2nd gas effect"" and ""reversed concentration effect"". METHODS: We studied 12 patients(6 female, 6 male:aged 30-50 yr) undergoing inhalation anesthesia with enflurane 1.5%-N2O 60%. In phase 1, after establishing a stable baseline(inspired enflurane concentration:1.2%, end-tidal enflurane concentration:1.0%), enflurane was stopped and the inspired gas was maintained as 60% N2O and 40% O2. In phase 2, enflurane 1.5%-N2O 60% administration was reinitiated, and when a stable baseline was obtained, enflurane was stopped, and the inspired gas was maintained as 60% N2 and 40% O2, then the end-tidal and inspired concentration of enflurane, end-tidal carbon dioxide tension and expired tidal volume were measured for a period of 5 minutes. RESULTS: The end-tidal concentration of enflurane was not significantly different between phase 1 and 2 at each time, but the inspired concentration of enflurane in phase 1 was significantly lower than that in phase 2 until 30 seconds after cessation of enflurane administration. The expired tidal volume in phase 2 was increased from 656 81.7 ml to 711.8 95.3 ml in 150 seconds after cessation of enflurane administration maximally, and sustained high level until 5 minutes compared to phase 1(P<0.05). But the end-tidal carbon dioxide tension was not significantly different between phase 1 and 2. CONCLUSIONS: For a period of 5 minutes after inhalation anesthesia with enflurane 1.5%-N2O 60%, the nitrous oxide elimination increases expired tidal volume, but not accerelates the elimination of enflurane, and not decreases end-tidal carbon dioxide tension both by the effect of inspired nitrous oxide concentration and by the difference in the metabolic rate and the brain/blood partition coefficient between halothane and enflurane.