This study was undertaken to determine the hemodynamic effects of metocurine in 40 adult patients (ASA class I-II) scheduled for elective operation. Anesthesia was induced with thiopental sodium 3-5 mg/kg IV and maintained with enflurane at a constant inspired concentration of 1.5-2.5 vo1%. After 30-minutes of hemodynamic stabilization and when operative stimulus was judged to be unchanged, each patient was randomly assigned to one of following three groups. Group I: Bolus injection of metocurine(0. 28 mg/kg, ED95) within 5 seconds. Group II: Slow injection of metocurine(0.28mg/kg, ED95) within 3 minutes. Group III: Metocurine injection as group I with prophylactic use of H(1)-H(2)-receptor blocker(Cimetidine 300 mg PO, 1 hour before induction and chlorpheniramine 1. 0 mg/kg IV, 10 min. before metocurine injection). There were no significant changes in mean arterial pressure and heart rate throughout the study (p< 0.05), but only one case of group I revealed the evidence of systemic histamine release. Group II and Group III showed no evidence of systemic histamine release. Therefore, we felt that metocurine might be used safely with slow injection or prophylactic use of H(1) and H(2) receptor blocker with rapid bolus injection for any surgical operation, including cardiovascular operations.