Previous studies have suggested that a negative component (N2) of the event-related potential (ERP), whose peak latency is 200-300 msec after stimulus onset, may vary in amplitude depending on the neuronal activity required for response inhibition. To confirm this, ERPs were recorded in a Go/No-go paradigm in which subjects of one group (HI, n = 10) were asked to respond to Go stimuli with key pressing within a shorter period (less than 300 msec) than those of the other group (LI, n = 10) whose upper limit of the reaction time was relatively longer (less than 500 msec). All subjects had to withhold the Go response to the No-go stimuli without making overt muscle activities. The N2 component was recorded superposed on the initial descending limb of the P300 and other slow deflections, which were attenuated with a digital filter to measure the amplitude of N2. The N2 amplitude was significantly larger to the No-go stimulus than to the Go stimulus in both groups, but the N2 to the No-go stimulus was significantly larger in the HI group than in the LI group. These differences in N2 amplitude between conditions or groups were thought to be independent of other ERP components such as P300 and CNV. These results suggest that at least to some extent N2, which increased in amplitude when a greater effort was required to withhold the Go response, reflects the activity of a response inhibition system of the brain.