Prolongation of the QT interval may produce potentially hazardous arrhythmias. The effects of midazolam or propofol, followed by suxamethonium on the QT interval have been investigated. Thirty patients, ASA I or II, without cardiovascular disease, electrolyte abnormalities or receiving any medication were studied. All patients were premedicated with midazolam 0.08 mg kg−1 i.m., 30–60 min prior to surgery. Anaesthesia was induced with either midazolam 0.4 mg kg−1 i.v., (15 patients) or propofol 1 mg kg−1 i.v. bolus, followed by a continuous infusion (15 patients). ECG, heart rate and arterial pressure were measured before induction of anaesthesia and at 1 and 3 min after the bolus of midazolam or propofol. Further recordings were obtained at 1 and 3 min after a bolus of suxamethonium 1.5 mg kg−1 i.v. and also immediately after tracheal intubation and 1 min later. Neither midazolam nor propofol had any significant effect on QTc interval. Prolongation of QTc was observed in both groups after suxamethonium and this was further prolonged following intubation. Heart rate and arterial pressure were also increased significantly in both groups following intubation.