The anterior cingulate is believed to play a crucial role in the regulation of thought and action. Recent evidence suggests that the anterior cingulate may play a role in the detection of inappropriate responses. We used event-related functional magnetic resonance imaging techniques to examine the neural responses to appropriate (correct rejects and correct hits) and inappropriate (errors of commission) behavioral responses during a go/no-go task. Analyses of the inappropriate responses revealed extensive activation in the rostral anterior cingulate cortex and in the left lateral frontal cortex. These areas were not activated for correctly classified trials (correct rejects and correct hits). These data suggest that the rostral anterior cingulate and left lateral frontal cortex are integral components of the brain's error checking system.