Movement-related potentials (MRPs) were recorded from subdural electrodes chronically implanted in the interhemispheric fissure in two patients being evaluated for epilepsy surgery. Different types of movements (finger, foot, tongue and vocalization) were executed. Foot movements elicited a clearly defined, well-localized slow negativity or positivity (Bereitschaftspotential, BP) preceding electromyogram (EMG) onset. These BPs were seen from the contralateral primary motor foot area and also from bilateral supplementary motor areas (SMAs) with equivalent amplitudes and temporal evolutions. A steeper potential [negative slope (NS')] occurred about 300 ms before EMG onset and the motor potential (MP) started 100 ms before EMG onset. Negative slopes and MPs also arose from the contralateral primary motor area as well as from the bilateral SMAs. Finger movements elicited well-localized BPs and NS' which were generated from the bilateral SMAs, but were of higher amplitude on the contralateral SMA. Motor potentials started 50 ms prior to EMG onset and arose exclusively from the contralateral SMA. Tongue protrusions and vocalizations also elicited BP, NS' and MP which were seen in the bilateral SMAs. Movement-related potentials for different types of movements had a somatotopic distribution in the SMA, which was consistent with the SMA somatotopic organization defined by electrical simulation. Movement-related potentials for tongue movements and vocalization had a similar distribution and waveform. It was concluded that bilateral SMAs generate well-defined MRPs consistent with the assumption that the SMA plays a significant role in the organization of voluntary movements. However, the MRPs from the bilateral SMAs do not have characteristics which are different from those of the primary motor area. This suggests the hypothesis of 'supplementary' function for SMA, and does not support the hypothesis of 'supramotor' function.