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Background: Patients undergoing a cesarean section under general anesthesia are at risk of intraoperative awareness due to the use of low concentration of volatile anesthetics used. This study investigated the effect of different methods for administering isoflurane on the anesthetic adequacy using bispectral index (BIS) in the early period of cesarean section. Methods: Eighty-two parturients undergoing a cesarean section were randomly assigned to receive 1 vol% isoflurane (Group EQ), 1.5 vol% isoflurane for the first 5 minutes and 1 vol% for the next 5 minutes (Group CO), 0.6 vol% end-tidal isoflurane immediately after intubation (Group ET). Thiopental 4 mg/kg was used to induce anesthesia. The bispectral index value, systolic and diastolic arterial pressure, heart rate and end-tidal concentration of isoflurane were recorded every minute for 10 minutes after intubation. Results: There were no significant differences in the systolic and diastolic pressure, and heart rate between the groups. The BIS values from 1 to 8 minutes after intubation were significantly lower in the ET group than in the EQ group. BIS values from 4 to 8 minutes after intubation were significantly lower in the CO group than in the EQ group. Conclusions: The administration of volatile anesthetics using the end-tidal concentration after thiopental induction shows the best anesthetic efficacy in the early period of cesarean section. This method may further reduce the level of intraoperative awareness. (Korean J Anesthesiol 2007; 52: 143~9)