A slow magnetic field shift has been detected in the human brain occurring in the foreperiod of a voluntary finger movement. This magnetic field accompanies a slow negative electrical cerebral potential which occurs in the same foreperiod, the Bereitschaftspotential (BP) of Kornhuber and Deecke. The present report is the first of a magnetic field associated with the BP, and has been named the Bereitschaftsmagnetfeld (BM) or readiness magnetic field. The BM is oriented with the field lines directed out of the head in the pre-rolandic region and with the field lines directed into the head in post-rolandic areas, suggesting a source in the sensorimotor area for the contralateral hand. Distribution of the magnetic fields has so far not revealed a source in the fronto-central midline where the BP is recorded maximally. The time course and morphology of the BP and BM are similar, but they have different topography over the skull.