We have recorded cerebral magnetic fields to trains of 40 Hz clicks in 6 subjects to study the neural generators of the 40 Hz auditory evoked response (AER). The largest deflection, N100m (magnetic counterpart of the electric N100), was preceded by a low voltage P40m and followed by P200m. A sustained field, SF, was seen at 300-500 msec. A rhythmic 40 Hz fluctuation was superimposed on the whole response. All deflections reversed polarity between the approximated ends of the sylvian fissure and the field patterns were dipolar during the peaks. The field patterns of the 40 Hz response suggested activation of the auditory cortex at the supratemporal plane, 2-3 cm beneath the scalp, close to the sources of P40m, N100m and SF. The equivalent source of SF was 0.5-1.5 cm anterior to the source of N100m in all subjects. The results suggest a cortical origin for the magnetic 40 Hz response, also suggesting that the electric 40 Hz AER is, at least in part, generated within the auditory cortex.