The regulation of firing thresholds of cortical neurons was suggested as one of the mechanisms underlying the generation of the P300 component in the human event-related potential. According to this hypothesis, the detection of an important stimulus produced the widespread inhibition of ?irrelevant? networks, interrupting their ongoing activity and facilitating the analysis of selected information. In the present experiment, the responsiveness of visual cortex was evaluated during the P300 potential by using additional, probing stimuli. Large separation of the cortical visual fields permitted separate analysis of the input and more advanced stages of processing. Responses were recorded from Fz, Cz, Pz and Oz scalp sites. P300 waves were evoked by visual, mentally counted stimuli in a standard 'odd-ball' procedure. Visual probes were delivered 200, 300, 400, 500, 700 and 1000 ms later. No responses to the probes were required. Significant suppression of responses to the probes delivered less than 400 ms after target stimuli was found in Oz and Pz but not in Cz or Fz . The suppression was not proportional to the voltage levels from which probe responses started. In Fz and Cz, latencies of probe responses were elongated if probes were delivered less than 400 ms after target stimuli. The results suggest that probe responses suppressed by the P300 potential in occipital and parietal cortex may be restored in frontal areas. In these areas the P300 potential could delay probe responses instead of suppressing them.