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.