Background: The purpose of this study was to investigate whether muscle relaxant affect the values of Entropy, response entropy (RE) or state entropy (SE) during propofol anesthesia. Methods: Eighty patients (ASA I) scheduled for elective surgery under general anesthesia were randomly assigned to four groups. Anesthesia was maintained at a SE value of 80 (80±2) using target controlled infusion (TCI) of propofol. After maintaining SE 80 for 5 min, vecuronium 0.1 mg/kg was injected intravenously in group I and same volume of normal saline was intravenously injected in group II. After maintaining SE 60 for 5 min, vecuronium 0.1 mg/kg was injected intravenously in group III and same volume of normal saline was injected intravenously in group IV. The mean arterial pressure, heart rate, SE and RE were measured before anesthetic induction and up to 5 min after vecuronium or normal saline injection in each group. Results: SE and RE were not changed in group II, but significantly decreased in group I (P＜0.05, respectively). In group III and IV, SE and RE were not changed in both groups. There were no significant hemodynamic changes among the four groups. Conclusions: These results suggest that the effect of muscle relaxant on Entropy vary according to the baseline values of RE or SE during propofol anesthesia. (Korean J Anesthesiol 2009;56:381~6)