We studied EEG reactivity to visual stimuli during deep isoflurane (1.5-2.05 vol% end-tidal concentration) anaesthesia. Twelve patients were anaesthetized with isoflurane until burst suppression occurred in the EEG. Red LED goggles were used to give visual stimulation of 60 flashes, 4-ms duration each, at a frequency of 20 Hz. The stimuli, 3-strains of flashes, were given at random intervals. Both onset and offset of stimulation evoked bursts. The latency of visually evoked bursts was comparable with long latency evoked potentials, which are known to be related to cognitive processing. Our data showed that the central nervous system reacts strongly to photic stimulation during deep anaesthesia.