Objective To investigate whether there is a difference in occurrence of electrocardiogram changes suggestive of myocardial ischaemia between two different doses of oxytocin. Design Double-blind randomised controlled trial Setting University hospital in Sweden. Population A total of 103 healthy women undergoing elective caesarean section under spinal anaesthesia. Methods The participants were randomised to 5 or 10 units of oxytocin, given as an intravenous bolus. A Holter monitor was used to record electrocardiograms and non invasive blood pressure and heart rate (HR) was monitored. A blood sample was obtained 12-hour postoperatively. Main outcome measures Depression of the ST segment. Secondary outcomes: symptoms, Troponon I levels, mean arterial pressure (MAP), HR and blood loss. Results There was a significant difference in occurrence of ST depressions associated with oxytocin administration, 4 (7.7%) with 5 and 11 (21.6%) with 10 units, P