The cortical electromyogram (EMG) activity, preceding voluntary movements, was recorded in 12 normal subjects in two different situations: first, when movements were self-induced by the subjects by their own will; and second, in response to threshold electrical stimulation of the index finger, a brief flash of a light-emitting diode (LED), and a click. Four types of movements were studied: (a) fast extension of the right wrist, (b) fast supination of the left wrist, (c) either movement depending on the subject's own decision or on which index finger was stimulated, and (d) fast sequential right and left wrist extension. In all subjects, self-initiated movements were preceded by a typical Bereitschaftspotential (BP) starting 1,290 +/- 208 ms before the EMG discharge. When the same movements were triggered by an external clue, there was no BP. The BP was present, although with a shorter duration, when subjects were asked to wait for a brief period after index finger stimulation, before extending the right wrist. From these results, we conclude that the BP is closely associated with the timing of internally generated movements, and that different cortical areas are probably involved in the generation of self-induced and externally referenced movements in humans.