The incidence of cardiac arrhythmias, heart rate, blood pressure, capillary perfusion and end-tidal CO2 tension were studied in 167 healthy children 1-12 years of age undergoing adenoidectomy (n = 82) and myringotomy (n = 85) during enflurane and halothane anaesthesia. The incidence of cardiac arrhythmias was significantly lower during myringotomy than during adenoidectomy. In children undergoing adenoidectomy the incidence of arrhythmias was 38.9% during enflurane anaesthesia and 86.6% during halothane anaesthesia (P less than 0.001). In the halothane group ventricular arrhythmias were observed in 19 patients (41.3%) but only in one child (2.8%) in the enflurane group. The ventricular arrhythmias seen during halothane anaesthesia were unifocal in six patients and multifocal in five and classified as ventricular tachycardia in eight children. Heart rate was increased by about 40% at the onset of ventricular arrhythmias. The heart rate remained unchanged with enflurane anaesthesia during surgery, which may reflect a decreased sympathomimetic activity. It is suggested that the low incidence of ventricular arrhythmias during enflurane anaesthesia may be explained by the combination of a reduced sympathomimetic activity and a lowered susceptibility of the myocardium to the actions of endogenous catecholamines.