Pre-event potentials were compared in the same subject, for 3 types of forewarned events, in which the foreperiod for orienting or attention began several seconds before the event. All of these trials involved similar non-motor components (expectancy, attentiveness, general orienting to a salient stimulus) but differed in whether motor or non-motor responses were required. A prominent and consistent slow negative shift preceded the pre-set time for a motor response, even when the subject 'vetoed' his intention to act shortly before the pre-set time. Pre-event potentials were absent, or small and brief, when the event was a task-related skin stimulus not involving preparation to move. The findings selectively support the view that mental preparation/intention to act is a necessary and perhaps dominant process associated with the vertex-recorded pre-event, slow negative potential. They also show that such a pre-event potential can appear even when the subject knows he is going to veto his developing intention to act and does not actually move.