A prolonged electrocardiographic QT interval may be harmful during general anaesthesia. It may be prudent, therefore, to select anaesthetic agents which have the least effect on the QT interval. In a controlled study, propofol has been shown to have less effect on the QT interval than thiopentone (P less than 0.05). Our data suggest also that any effects which may be caused by enflurane and isoflurane are masked by the effects of the induction agent.