SUMMMARY Ten right-handed patients with Parkinson's disease and 9 normal subjects performed five different types of movements as rapidly as possible in their own time. (1) isotonic elbow flexion through an angle of 15 deg (‘flex’); (2) isometric squeezing of a force transducer between thumb and fingers (‘squeeze’); (3) isotonic finger flexion (‘cut’); (4) simultaneous performance of both ‘flex’ and ‘squeeze’ (5) simultaneous performance of ‘flex’ and ‘cut’. The patients performed the separate movements of ‘flex’, ‘squeeze’ and ‘cut’ more slowly than normals However, a more striking deficit was lseen when a ‘flex’ and a ‘squeeze’ had to be performed at the same time, and with the same arm. There was an additional increase in movement times over and above that seen in the separate movements alone. If the patients used both arms (‘flex’ with the right, ‘squeeze’ with the left), rather than one, or when a ‘flex’ and a ‘cut’ had to be combined in the same arm, only a slight increase in movement times was observed. In normals, however, the speed of individual movements of ‘flex’, ‘squeeze’ or ‘cut’ was the same irrespective of whether they were performed separately or simultaneously In any one subject, movement times for the separate components of ‘flex’ and ‘squeeze’ vaned independently dunng the performance of the simultaneous movement. Because they remain independent, we suggest that when ‘flex’ and ‘squeeze’ are performed at the same time, two separate motor programmes are superimposed to produce the combined movement. In Parkinson's disease there may be a deficit in superimposing two separate motor programmes which leads to the pronounced slowness of simultaneous movements with the same arm. Comparison of movement times for a ‘flex’ (but not for a ‘squeeze’) in the separate and simultaneous movements showed that the degree of clinical akinesia was more closely related to the additional slowness in simultaneous movements than to the slowness seen when the movements were performed separately. The degree of disturbance in superimposing separate motor programmes may determine the amount of clinical akinesia in patients with Parkinson's disease