A method of chronological data storage and reverse computation is described by which bio-electrical phenomena preceding 'spontaneous' events within the nervous system can be analysed if these events appear repeatedly and are capable of triggering a computer.Slow brain potentials accompanying voluntary and passive movements of the limbs were analysed by this method. These potentials were recorded from different points of the scalp from 12 healthy subjects in 94 experiments with more than 100 movements in each record. At times artifacts were superimposed upon cerebral potentials. The former were identified, and, as far as possible, eliminated.Voluntary hand or foot movements are preceded by a slowly increasing surface-negative cortical potential of 10-15 μV, called readiness potential. This potential is maximal over the contralateral precentral region, but shows bilateral spread and is larger over the frontal than over the occipital areas. The readiness potential increases with intentional engagement and is reduced by mental indifference of the subject.Voluntary movements are followed by a complex potential with an early positive phase that begins 30-90 msec after the onset of movement. The late potentials following voluntary movements are similar to those after passive movements. Both resemble the late bilateral components of the evoked potentials after electrical stimulation of peripheral nerves. Some variable differences between the early components of the potentials after the onset of active and passive movements require further investigation.No relation between the onset of voluntary movements and the phase of the alpha rhythm could be detected.Further applications of reverse computation are addressed. Similarities between the readiness potential and G. WALTER'S expectancy wave in conditioned reflexes, and the sources of artifacts through eye movements etc. are discussed.