Sixteen subjects were used to study the effectsof two levelsof pacing constraint and four performance feedbacks on the performance and mental load associated with a manual task. The following main conclusions emerged: (I) performance feedback reduces the mental load of the task only when the pacing constraint is low and (2) feedback improves task performance when the pacing constraint is low since operators are able to meet work requirements more closely. Feedback, however, does not improve the performance or reduce the mental load of a highly paced task.