Activity of the supplementary motor area may be inferred from movement-related potentials (MRPs) which are associated with the preparation and execution of voluntary, or internally determined movements. Supplementary motor area activity may be abnormal in Parkinson's disease since its major input from the basal ganglia is disrupted. Investigation of the abnormalities in supplementary motor area activity associated with movement deficits in Parkinson's disease may therefore reveal functions of the basal ganglia and the supplementary motor area. Movement-related potentials associated with sequential movements were investigated under various cueing conditions in Parkinson's disease subjects and age-matched controls. In controls, MRPs revealed involvement of the supplementary motor area in movements which can be internally determined (non-cued and externally cued, predictable movements, but not unpredictable movements). In Parkinson's disease, however, the supplementary motor area was only involved in movements which must be internally determined (non-cued movements, but not externally cued movements); therefore impaired internal control mechanisms, operating via the supplementary motor area, are bypassed when external cues are given. As a result, Parkinson's disease patients are more reliant on external cues and are unable to use predictive models to internally guide movement. Supplementary motor area involvement also relied on the predictability (in controls) or presence (in Parkinson's disease) of timing cues and not spatial cues, indicating a role of the supplementary motor area and basal ganglia in the temporal organizations of sequential movement rather than the programming of specific movements. For non-cued movements, abnormalities in MRPs for Parkinson's disease subjects consisted of delayed MRP onset and peak times, and prolonged cortical activity following movement. These observations led to a proposed model of the interaction between the basal ganglia and the supplementary motor area, involving the temporal organization of voluntary and internally determined sequential movements.