Halothane, in common use today, sensitizes the myocardium to endogenous and exogenous sympathomimetic amines arid induces cardiac arrhythmia, sometimes life threatening. Sympatbomimetic amines, however, are frequently injected subcutaneously for hemostasis or intravenously far cardiovascular stability. Therefore, this study was performed to investigate the effect of lidocaine 1 mg/kg, pro-pranolol 0.02 mg/kg, and droperidol 0.1 mg/kg pretreatment on arrhythmias(A.R.) and changes in heart ramie(H.R.), systolic bood pressure(T.B.P ) and diastolic blood pressure (D.B.P.) Induced by Ⅳ administered ephedrine 0.2 mg/kg. Patients were divided into 5 groups: 20 cases without pretreatment(Group I ), 10 cases with lidocaine prtreatment(Group Ⅱ), 10 cases with propranolol pretreatment(Group Ⅲ), 20 eases with lidocaine-propranolol pretreatment(Group Ⅳ) and 20 cases with droperidol pretreatment(Group V ). The results were as follows: 1) In Group Ⅰ, ephedrine produced A.R. in 16 cases(80%) and significant increase in H.R.(d∼11 bpm, p<0.001) and S.B.P. (8∼22 torr, p<0.001), but D.B.P. increased insignif-icantly(2∼10 torr, NS). 2) In Group Ⅱ, ephedrine produced A.R. in 5 cases(57%) an? 5.B.p.(10∼17 terr, p<0, 01) increased significantly, but H.R. remained unaltered. 3) In Group Ⅲ, ephedrine produced A.R. in 3 cases(30%) and H.R. (6∼8 bpm, p<0.05) decreased, but S.B.P.(12∼21 torr, p<0.01) and D.B.P (8∼16 torr, p<0.01) increased signi-ficantle. 4) In Group Ⅳ, ephedrine produced A.R. in 2 cases(10.%) and H.R.(3∼6 bpm, p<0.05) decreased, but S.B.P (4∼11 torr, p<0.05) and D.B.P.(3∼9 torr, p<0.01) increased signific-antly. 5) In Group V. ephedrine produced A.R. in 2 cases(10%) and H.R.(9∼13 bum, p<0.001) increased siginificantly, but S.B.P. and D.B.P. remained unaltered. From the above results, it is concluded that lidocaine and propranolol mixture or droperidol protects most effectively against ventricular arrhythmias induced by ephedrine during halothane-N2O anesthesia in human volunteers.