BACKGROUND: An increase in ventilation is known to accelerate the rate of the rise of alveolar concentration of inhalation anesthetics and increase the rate of anesthetic induction. However, it is still debatable whether or not a rise of alveolar isoflurane concentrations by increased minute ventilation (MV) actually increases the depth of anesthesia. METHODS: After anesthetic induction by intravenous thiopental and succinylcholine, and after the bispectral index (BIS) had recovered to 75 after intubation, isoflurane anesthesia was begun (inspired concentration:2 vol.%, respiratory rate:12/min, tidal volume: group I; 8 ml/kg, group II; 12 ml/kg). BIS, spectral edge frequency (SEF), blood pressure, pulse rate were recorded 1 minute before induction, and 1, 2, 3, 4, 5, 7 and 10 minutes after isoflurane anesthesia. End-tidal concentrations of isoflurane and CO2 (ET-iso and -CO2) were recorded at 1, 2, 3, 4, 5, 7 and 10 minutes after isoflurane anesthesia. RESULTS: The increase of ET-iso and the decrease of ET-CO2 were significantly fast in group II. There was no difference in BIS and SEF between the two groups. CONCLUSIONS: Increasing MV induced a rapid rise of alveolar isoflurane concentrations, but changes of BIS and SEF were not affected by increased MV.