We have examined the raw EEG activity and auditory evoked responses (AER) in 35 children, aged 3 days to 13 yr (median 1.5 yr), undergoing general anaesthesia for routine surgery. Binaural stimuli were presented at a frequency of 6.12 Hz and the EEG was recorded and stored using the Northwick Park auditory evoked response software. AER were generated by averaging 512 sweeps each of 125-ms duration. In children less than 2 yr of age, the AER was often irregular, whereas children older than 2 yr had AER patterns similar to those in adults. In children less than 2 yr, regular artefact activity was superimposed on the background EEG which was at the same frequency as the instantaneous heart rate and which was often identifiable as the ECG. We conclude that the AER may be unreliable in children less than 2 yr of age, and modification of current methodology may be required if this technique is to become useful in paediatric anaesthetic practice.