BACKGROUND: There is difference between isoflurane and enflurane in the myocardial sensitization to catecholamines, and their actions on the heart. Enflurane reduces cytoplasmic calcium more than isoflurane. The hypothesis could be suggested that these two volatile anesthetics might cause the different incidences and natures of cardiac arrhythmias. METHODS: The anesthesia was induced by the intravenous injection of thiopental(6 mg, kg-1) and pipecuronium(0.1 mg, kg-1). Two groups were randomly allocated to the patients(n=80) in the maintenance: Group I inhaled isoflurane(1~2%), O2(2 L), and N2O(2 L), Group II, enflurane (1.5~2.5%). Continuous electrocardiographic recordings with Holter monitor were made of those undergoing tympanoplasty during anesthesia. The tapes recorded were scanned using analyzer, and we read out ECG complexes on the screen. Results were categorized as induction, maintenance, and emergence, and inferred from unpaired t-test, and X2-test with p<0.05 considered significant. RESULTS: The results were as follows: 1) The total incidence of cardiac arrhythmias was 65.0%. Isoflurane(52.5%) was lower than enflurane(77.5%) in the incidence. 2) During maintenance and emergence, isoflurane was lower than enflurane in the frequency of supraventicular arrhythmias. 3) Ventricular arrhythmias most frequently occurred in induction. 4) The frequency of supraventricular arrhythmias was more than two times that of ventricular ones. The arrhythmias at the AV junction were the most common, and VPC's the second. CONCLUSIONS: Greatest caution should be paid during peri-induction. It could be suggested that compared to enflurane, isoflurane better be administered for those to whom arrhythmias could be harmful.