This study was undertaken as a further evaluation of spontaneous facial EMG activity as an index of mental effort. We investigated whether concordant alterations in task performance level and EMG amplitude existed during a sustained information processing task. The EMG of six different facial and jaw-elevator muscles was recorded in 21 subjects performing a 20 min externally paced visual two-choice serial reaction task and in 24 other subjects performing a self-paced version of this task. In both conditions, a post-hoc division was made between subjects with stable task performance parameters and subjects with a decline in performance throughout the task period. In all subject groups, there was a gradual increase in EMG activity of frontalis, corrugator, and orbicularis oris inferior muscles following task onset. As in earlier studies, this increase was interpreted as a sign of growing compensatory mental effort. In the subject groups with declining performance, however, the initial EMG increase passed into a decreasing trend towards the end of the task period whereas in the groups with stable performance, EMG increased uninterruptedly. These results were interpreted as further support for the hypothesis that EMG activity in particular facial muscles is related to the mobilization of aspecific energetic resources.