Backgrounds: The purpose of this study was to evaluate the effect of ephedrine and phenylephrine on propofol concentrations and bispectral index during propofol anesthesia. Methods: General anaesthesia was induced with propofol and was maintained with propofol (4 mg kg−1 h−1) and fentanyl. Vecuronium was used to facilitate the artificial ventilation of the lungs. Patients with systolic blood pressure > 90 mmHg were defined as the control group (n = 16). Patients who had to be treated for larger decreases in arterial blood pressure (systolic blood pressure <90 mmHg) were randomly assigned to receive ephedrine 0.1 mg kg−1 (n = 20) or phenylephrine 2 μg kg−1 (n = 20). Propofol concentrations were determined before (baseline) and at 3 and 10 min after the administration of ephedrine or phenylephrine. Bispectral index was monitored continuously. Cardiac output was determined using a pulmonary arterial catheter. Results: Ten minutes after drug injection, bispectral index in the ephedrine group increased significantly compared with the baseline and was significantly larger than that in the control or phenylephrine group. Eight patients in the ephedrine group had bispectral index >60, whereas no patient in the control or phenylephrine groups had bispectral index >60. There were no significant differences in propofol concentrations or cardiac output relative to baseline at 3 or 10 min after the administration of ephedrine or phenylephrine. Conclusions: Ephedrine increases bispectral index values without decreasing propofol concentrations during general anesthesia.