The function of the anterior cingulate cortex (ACC) has been discussed in the last years in the context of conflict monitoring and error detection. In addition, ACC activity has been described in the context of "conscious effort". Recent neurophysiological and neuroimaging studies have described a negative correlation between ACC activity and reaction times in simple or choice reaction time experiments. One suggested explanation for this finding has been that there is a relationship between effort and ACC activity. The present ERP-LORETA study of healthy volunteers (n=35) was intended to directly investigate this relationship. In this experiment, three conditions were investigated: condition I was a choice reaction task with the instruction to stay relaxed during the task (relaxed condition), condition II was the same choice reaction task with the instruction to press the respective button as fast and correct as possible (effort condition). Condition III was just listening to the tones without button press (control condition). Subjects had to score directly after each experimental run on a visual analogue scale the amount of effort they have actually spent. The subjects showed significantly shorter reaction times during the high effort condition in comparison to the relaxed condition, as well as increased N1 amplitudes and increased ACC activity. In a subgroup analysis, this effect was present only in subjects who were (according to their self-ratings) following the instructions closely. These results provide direct evidence for a close relationship between conscious effort and ACC activity and suggest the usefulness of the applied effort-self-rating.