The anticipatory (preparatory) cerebral events in simple reaction-time tasks may depend on the ability of a subject to predict accurately the time of occurrence of the stimulus requiring a particular response. In order to examine this hypothesis, we recorded cerebral and muscle responses in two different conditions, each involving three simple reaction-time tasks. Auditory tones were presented either regularly (i.e., predictably) or irregularly and subjects were required to respond to each tone with left hand, right hand or both hands in different runs. Responses were simultaneously averaged both backward and forward from the response (response-synchronized) and forward from the stimulus (stimulus-synchronized). Response-synchronized cerebral potentials to the regular tones were characterized by a slow negative shift, the bereitschaftspotential (BP), that began prior to stimulus onset and whose terminal phase was characterized by a small, higher frequency, negative shift (HFNS). By contrast, response-synchronized cerebral potentials to the irregular tones for both groups did not contain a BP, but a more conspicuous HFNS that began after stimulus onset. Both the response time and the latency of the N1 sensory-related component of the cerebral evoked potential recorded in the stimulus-synchronized averages (which aligns with HFNS) were delayed in the irregular condition. These findings suggest that, for both right- and left-handed subjects, the BP is not required for voluntary movement, and that anticipatory cerebral activity, as reflected by the BP, represents not only a preparation to make a particular response but also a preparation to process the stimulus.