The EEG activity preceding self-paced voluntary movements (movement-related cortical potential, MRCP) is smaller if subjects make the same movement each time (regular task) compared with when different movements are made each time (random task). To test whether extra activity in the random task is due to increased motor preparation needed to switch between different movements, or to memory/attentional processes needed to select movements randomly, we compared regular and random movements with an additional alternating task. This alternating task required subjects to make different movements each time as in the random task, but since the task was very simple, the memory/attentional load was similar to that in the regular task. The MRCP was equally large over motor areas in both random and alternating tasks, suggesting that the extra activity over sensorimotor areas reflected processes involved in motor preparation rather than memory/attention. We speculate that, in the regular task, some part of the instructions for the previous movement remains intact, reducing the amount of preparation needed for the next repetition. Thus the MRCP is smaller than in the alternating and random tasks. Although the MRCPs in alternating and random tasks were similar over the motor areas, the random task had more activity than the alternating task in contralateral frontal areas. This part of the MRCP may therefore be related to memory/attentional processes required to randomize the sequence of movements. We conclude that the MRCP contains dissociable components related to motor preparation and memory/attention.