Recent results suggest that a negative ERP potential emerges not only on error trials, but is also found after correct responses. There is broad evidence that this component is generated in the anterior cingulate cortex (ACC). The present study aimed to explore the influence of working memory demands on the evaluation process probably reflected by the negative potential. To this purpose, a modified continuous performance task (CPT) was used, with variations of the delay between cues and imperative stimuli. Data were analyzed using conventional averaging techniques as well as source localization with LORETA (low resolution brain electromagnetic tomography). Results suggest a significant effect of working memory delay on the amplitude of the post-response negative ERP component. Its source was located in the anterior cingulate cortex, with the exact location being dependent upon working memory demands. The results support the notion of a general response evaluation system reflected by a post-response negative component. The findings of a working memory dependent modification of this potential suggest a functional link between the medial frontal cortex and the lateral frontal regions primarily involved in working memory processing.