Practice effects on the preprogramming of discrete movements in Parkinson's disease. The effects of practice on the simple and choice reaction times (RTs) of Parkinson's disease (PD) and control subjects in a discrete aiming task were analysed. For controls, practice led to a selective decrease in choice RTs, as has been reported previously. An opposite effect was seen in the PD group, with little change in choice RTs and substantial reduction in simple RTs. The results suggest that PD subjects can use advance information to initiate discrete movements more rapidly, but that this ability to "preprogramme" movements requires practice. Reconciliation of these results with studies reporting an inability to preprogramme in PD are made in a discussion of task characteristics which may allow or preclude preprogramming. 