Objective To investigate the effects of different target plasma concentrations of propofol given by TCI on end-tidal isoflurane concentration when the depth of anesthesia was maintained at BIS 50 and evaluate the reliability of electromyography (EMG) as an anesthesia depth monitor. Methods Sixty ASA Ⅰ-Ⅱ patients aged 40-65 yrs weighing 40-85 kg undergoing elective abdominal surgery under general anesthesia were randomly allocated to one of three groups with 20 patients in each group : group Ⅰ isoflurane; group Ⅱ isoflurane + TCI propofol (1 ?g?ml-1) and group Ⅲ isoflurane + TCI propofol (2 ?g?ml-1). The patients were premedicated with intramuscular phenobarbital 0.1 g and scopolamine 0.3 mg. Anesthesia was induced with midazolam 0.05 mg?kg , fentanyl 3 ?g?kg-1 and propofol 0.5-1.0 ?g?kg-1. Tracheal intubation was facilitated with vecuronium 0.12 mg?kg-1. After intubation anesthesia was maintained with isoflurane inhalation alone (group Ⅰ) or isoflurane combined with TCI propofol at a target plasma concentration of 1 ?g?ml-1 ( group Ⅱ) or 2 ?g?ml-1 ( group Ⅲ). MAP, HR, SpO2, PETCO2, BIS, EMG and end-tidal isoflurane concentration were continuously monitored during anesthesia. BIS was maintained at 45-55 after tracheal intubation during maintenance of anesthesia.Results The three groups were comparable with respect to age, sex, body weight, duration of anesthesia and the total amount of fentanyl used during anesthesia. There were no significant differences in MAP, HR and SpO2 among the 3 groups. When BIS was maintained at 50, the end-tidal isoflurane concentration was 0.76?0.03% (group Ⅰ), 0.43? 0.08% (group Ⅱ) and 0.21?0.07% (group Ⅲ) respectively. EMG was maintained at 26-29 during operation. During emergence from anesthesia EMG value increased with BIS value. When the patients opened their eyes at command EMG value was about 40. The correlation between BIS and EMG was poor. Conclusion At the samedepth of anesthesia (BIS = 50) the end-tidal isoflurane concentration was greatly reduced when combined with TCI propofol. EMG value decreases with increasing depth of anesthesia but as an anesthesia depth monitor it still needs improving.