The amplitude and duration of the Bereitschaftspotential were analysed in 9 normal subjects prior to four different motor tasks: (1) simple isotonic elbow flexion ('flex'), (2) simple isometric finger flexion ('squeeze'), (3) simultaneous performance of 'flex' and 'squeeze', and (4) sequential performance of 'flex' then 'squeeze'. The Bereitschaftspotential was much larger and slightly longer in the simultaneous and sequential movements than in the same movements performed separately. The amplitudes of the Bereitschaftspotential were measured 150 and 75 ms prior to electromyographic (EMG) onset of the prime mover, and at the time of EMG onset. Changes in size were seen in all leads (FCz, Cz, C3', C4') and at all time points. It is suggested that the increase of the Bereitschaftspotential prior to complex arm movements reflects a greater activation of the supplementary motor area before simultaneous and sequential movements.