A method of accurate storage and on-line preprocessing of an EEG signal, preceding and following a trigger signal, elicited by button pressing, is described. The method was used to study the changes occurring in the power of the rhythmic activity within the alpha band in central areas, during voluntary, self-paced movement in 10 normal humans. A short-lasting decrease or phasic event-related desynchronization (ERD) of alpha power, representing mu activity, was observed in all 10 subjects. During the 2 sec period preceding movement the phasic ERD was mostly bilateral, but larger prior to right than to left thumb movement. At onset and during the first second of execution of movement, the phasic ERD was mostly bilateral but predominant in ipsilateral areas. Preceding or during movement, maximum ERD was observed in most cases in central-vertex regions.