QT interval was studied in 156 adults and in 127 children during the induction of anaesthesia. Both in adults and in children, QT interval was prolonged statistically significantly after thiopentone 5 mg/kg and the most marked prolongation occurred after suxamethonium 1 to 1.5 mg/kg. In adults, d-tubocurarine 0.06 mg/kg, but not alcuronium 0.03 mg/kg or pancuronium 0.01 mg/kg, prevented statistically significantly the prolongation of the QT interval after suxamethonium 1.5 mg/kg. In children, all three muscle relaxants prevented statistically significantly the effect of suxamethonium 2 mg/kg and pancuronium also prevented the effect of thiopentone. The most common ECG changes were ventricular ectopic beats (VEB) which occurred in 26% of the adults and in 22% of the children who were not pretreated with the muscle relaxants. After pretreatment with d-tubocurarine, the incidence of VEB was 3% in both groups. In adults, alcuronium was as effective as d-tubocurarine in the prevention of VEB but in the alcuronium group supraventricular ectopic beats and junctional rhythm occurred in 6% and 9% of the patients, respectively. Pancuronium did not significantly prevent the incidence of VEB. On the basis of the present results, d-tubocurarine is the relaxant of choice for the prevention of the prolongation of QT interval as well as ECG changes during the induction of anaesthesia.