Clonidine, an a2-adrenergic agonist, has sedative and analgesic properties and reduces the inhalation anesthetic requirement and modifies the hemodynamic responses to surgery. But occasionally, severe bradycardia and hypotensian during inhalation anesthesia develops following oral clonidine premedication. To predict intraoperative hemodynamic changes during enflurane anesthesia, author evaluated noninvasive autonomic tests consisted of respiratory sinus arrhythmia (RSA), Valsalva's maneuver (VSV), R-R interval difference in head up tilt (HTR) and diastolic blood pressure difference in head up tilt (HTP) in fifty-one patients (ASA physical status I~II ) scheduled for elective surgery. Patients were received 5 ug/kg of oral clonidine at 90 min before operation. Anesthesia was maintained with enflurane, oxygen (2 liter/min) and nitrous oxide (2 liter /min). The enflurane concentration was controlled to maintain blood pressure within +/-20% of preinduction value. After anesthesia, patients were allocated to two groups according to above and below 0.5 MAC (0.84 vo1%) enflurane concentration. Each preoperative autonomic test results revealed significant correlation with heart rate (HR), mean arterial pressure (MAP), and enflurane concentration (EC), respectively (p<0.05). Values of RSA, HTR, HTP, MAP and HH wm 81+/-14.4 ms, 76+/-43.5 ms, -6+/-5.2 mmHg, 76+6.4 mmHg and 63+/-5.5 beats/min, respectively in below 0.5 MAC enflurane concentration group and were significantly different from values of RSA; 16933.7 ms, HTR; 175+/-41.7 ms, HTP; 6+/-3.1 mmHg, MAP; 90+/-11.0 mmHg and HR; 76+/-7.6 beats/min in above 0.5 MAC enflurane concen- tration group (p<0.05). In conclusion, the simple noninvasive autonomic tests are recommended in prediction of intra-operative hemodynamic changes during enflurane anesthesia when oral clonidine premedication is indicated.