The correlation between electroencephalogram (EEG) and instantaneous heart rate was studied in deep isoflurane anaesthesia when the EEG showed a burst-suppression pattern in 15 patients aged 19-39 years. At the onset of EEG burst, the heart rate started to accelerate, and at the onset of suppression it started to decelerate. These changes in heart rate are similar to those we have previously described with enflurane, despite several differences in the effects of these two anaesthetics on EEG and on the cardiovascular system. We hypothesise that the heart-rate changes are due to central inhibition mediated by the vagus nerve during EEG suppression.