Monitoring one's actions is essential for goal-directed performance. In the event-related potential (ERP), errors are followed by fronto-centrally distributed negativities. These error(-related) negativity (Ne/ERN) amplitudes are often found to be attenuated in patients with Parkinson's disease (PD) compared to healthy controls (HC). Although Ne/ERN has been proposed to be related to dopaminergic neuronal activity, previous research did not find evidence for effects of dopaminergic medication on Ne/ERN amplitudes in PD. We examined 13 PD patients "on" and "off" dopaminergic medication. Their response-locked ERP amplitudes (obtained on correct [Nc/CRN] and error [Ne/ERN] trials of a flanker task) were compared to those of 13 HC who were tested twice as well, without receiving dopaminergic medication. While PD patients committed more errors than HC, error rates were not significantly modulated by dopaminergic medication. PD patients showed reduced Ne/ERN amplitudes relative to HC; however, this attenuation of response-locked ERP amplitudes was not specific to errors in this study. PD-related attenuation of response-locked ERP amplitudes was most pronounced when PD patients were on medication. These results suggest overdosing of dopaminergic pathways that are relatively spared in PD, but that are related to the generation of the Ne/ERN, notably pathways targeted on the medial prefrontal cortex.