Neuromagnetic fields accompanying voluntary flexions of the right index finger were studied in five subjects. In all subjects, slow magnetic fields were observed over the central scalp beginning about 1 second prior to movement onset. These fields displayed a similar time course to the electrically recorded "readiness potential", but with reversals of field direction over regions of the rolandic fissure over both hemispheres. Least-squares fitting of two current dipole sources for the pre-movement fields resulted in a consistent localization of one source in the region of the rolandic fissure contralateral to the side of movement in four subjects. Ipsilateral dipole sources fitted inconsistently at deeper locations or outside the head indicating the inability of a single dipole source to account for the ipsilateral fields. A large field reversal was also observed over the contralateral (left) hemisphere, 90-130 ms after onset of EMG activity in the active muscles. In some subjects, single dipole sources could be fitted to this "movement-evoked" field at locations slightly deeper and posterior to the pre-movement source locations in the contralateral hemisphere, possibly indicating unilateral activation of somatosensory cortex related to sensory feedback during the onset of this movement. Subtraction of pre-movement field activity from post-movement fields improved the ability to fit a single contralateral rolandic source for all subjects suggesting that pre-movement sources continue to be active during movement onset. These findings confirm previous reports that voluntary finger movements are preceded by slow magnetic fields.(ABSTRACT TRUNCATED AT 250 WORDS)