This study examined cortical motor structures that are involved in preprogramming and execution of movements. In two independent experiments a response precuing task was employed that combined the recording of movement-related brain potentials (MRPs) with spatio-temporal source localization. Behavioral and MRP results indicated the utilization of advance information about movement direction and hand. Dipole source modeling of foreperiod MRPs revealed a reliable three-dipole solution with sources located in lateral and medial brain regions anterior to the precentral gyrus. These dipoles were located in the lateral premotor area (PMA) and supplementary/cingulate motor areas (SMA/CMA). Activity of the medial dipole increased with the extent of advance motor preparation, whereas lateral dipole activity revealed parallel preparation of both response hands when only partial information about movement direction was available. The dissociation in the strength and the onset of medial and lateral dipole activity indicated two phases of motor preparation. We propose that medial motor areas like SMA and CMA are involved in the assembling and selection of abstract movement programs, whereas lateral PMA and primary motor cortex are involved in effector-specific motor preparation.