There is evidence from neuropsychological and psychophysical measurements that sensory information is processed in discrete time segments. The segmentation process may be described as neuronal oscillation at a frequency of 30-40 Hz. Stimulus-induced neuronal oscillations of this frequency are found in the middle latency range of the auditory evoked potential (AEP). We have studied the effect of different end-tidal concentrations of isoflurane on auditory evoked 30-40 Hz neuronal oscillations. We studied 13 patients undergoing intra-abdominal urological and gynaecological procedures. AEP were recorded in the awake state and during end-expiratory steady state isoflurane concentrations of 0.3, 0.6 and 1.2 vol%. These incremental doses of isoflurane caused a stepwise decrease in frequency of oscillations. The decrease in oscillation frequency and sometimes the disappearance of oscillatory components may be interpreted as suppression of sensory information processing. The measurement of auditory evoked neuronal oscillations in the AEP appears to be a promising tool to monitor both sensory information processing capacity and depth of anaesthesia.