To assess possible changes of error processing with mental fatigue, error-related brain potentials were recorded from young and older adults while they performed various cognitive tasks during several hours with normal breaks between task blocks. The EEG was recorded at 64 electrodes, and response-related ERPs were computed. Data from two tasks will be reported, a four-alternative choice task (4-CR) and a distractor task (flanker). Behavior was unaffected by time-on-task. The error negativity (Ne or ERN) a correlate of early mismatch resp. error detection, was slightly reduced with time-on-task, but only for the young subjects, and mainly so in the 4-CR task. Ne latency was prolonged with time-on-task, but only for the elderly, and only in the flanker task. The latency of the error positivity (Pe ), which depends on conscious error recognition, was prolonged with time-on-task for all subjects, but only in the flanker task. The reduction of the Ne is in line with data from sleep deprivation (Scheffers et al. 1999) and studies with prolonged uninterrupted work (Lorist et al. 2003). In sum, the present results suggest an impairment of different aspects of error processing with mental fatigue. Further work shoud assess the preconditions for such impairments.