Sequential postural adjustments associated with the task of rising on tip-toes were investigated in a reaction time paradigm in 30 normal subjects, 10 patients with Parkinson's disease and 2 patients with cerebellar lesions. The typical motor pattern observed in normal subjects consists of preparatory activity in tibialis anterior (TA, shifting the body forward) and quadriceps femoris (QUA, stabilizing the knee joint) followed by executional activity in triceps surae (TS) resulting in the movement itself. Shortening of TS without preparation would result in shifting the body backward and in flexion of the knee. The coordinated pattern of motor activities is obviously adapted to functional demands. Leaning forward or backward prior to the movement results in additional adaptive changes in the timing of preparatory versus executional EMG activity, and in the amount of preparatory muscle force. The basic pattern of preparatory (TA and QUA) and executional (TS) activity was preserved in most patients with Parkinson's disease. Reaction times were normal or only moderately delayed. Time intervals between postural preparation and execution were also normal. But trial to trial variation of reaction times was increased. In the two patients with cerebellar disorders, motor preparation in QUA was delayed beyond TS onset. EMG activity was tonic and cocontraction of antagonistic muscles occurred. The basic pattern of motor preparation and execution was again preserved. It must therefore be generated outside the basal ganglia and the cerebellum. The temporal coordination of muscular activity within each and between the two components and scaling of muscle force are under cerebellar control.