Sequencing in Parkinson's disease. Abnormalities in programming and controlling movement. Central programming deficits in Parkinson's disease (PD) were studied in two reaction time (RT) experiments. In Experiment 1, PD patients and controls performed sequences of hand postures that varied in length, the number of different postures (repetitive vs heterogeneous), and the delay interval before movement. Before movement, the PD group planned repetitive movements like controls whereas for heterogeneous sequences RT increased less with sequence length for the PD group, implying less preprogramming. The interresponse time (IRT) data from repetitive sequences showed that the PD group had difficulty controlling movement such that IRTs were faster when sequences were longer, thus allowing more time to schedule the termination of the sequence during the course of movement. For heterogeneous sequences, the PD group made more errors and were slower than controls when changing hand postures, suggesting a deficit in switching between different responses. While RT decreased with a longer delay similarly for both groups, IRT1 continued to improve only for the PD group but similarly for both types of sequences, suggesting a deficit specific to programming the first response. In Experiment 2, subjects made decisions about the number of different hand postures contained within a sequence. PD patients' decision times improved more with a longer delay only for heterogeneous sequences, suggesting a problem in identifying the number of different hand postures. The results have implications for levels of motor dysfunction in PD which emphasize the influence of sequence length and complexity. 