THE frontostriatal system appears to play a crucial role in the organization and execution of sequential behaviour, but the precise nature of its contribution remains to be specified. From this perspective, relatively simple modifications of behavioural task parameters may invoke rather profound changes in the recruitment of appropriate neural mechanisms, including the frontostriatal system. This mini-review examines how variations in task requirements for sequence learning and related cognitive tasks can induce significant modifications in the performance of patients with Parkinson's disease. In particular, these observations are used to support a developing argument for neurophysiologically dissociable sequence learning systems in man. One of these mechanisms is sensitive to surface structure, or element-by-element sequence organization, and appears to rely on the frontostriatal system. Another sequence learning mechanism is sensitive to abstract structure, or relationships between repeating sequence elements, and appears to be largely independent of the frontostriatal system.