Subgroups of clinically defined responders and non-responders to clomipramine or maprotiline showed different electroencephalographic dynamics of vigilance, even before pharmacotherapy was initiated. Two hours after the first drug administration subgroup differences no longer existed. On the 21st day of medication we found a tendency towards re-establishing the pre-drug situation. Our findings lend support to the assumption of 'distinct biochemical subtypes' which, however, rather correspond to different stages of the pathophysiological process than represent invariant 'traits'.