In 62 volunteers it was studied, whether an alternating 3 Hz magnetic field (induction 0.1 mT) vertically applied to the head over a period of 20 min causes changes in EEG parameters. The study's design was a random crossover controlled, blind one. The field was generated by a Helmholtz coils arrangement. The occipital surface EEGs (O1 and O2) were derived against the left earlobe. Significant differences (two-tailed P < 0.05) between sham and real exposure were found for the relative spectral amplitudes of the theta (3.5-7.5 Hz) and beta band (12.5-25.0 Hz) and the theta/beta ratio. These observations can be interpreted as a more pronounced reduction of alertness under the real field condition compared with the control.