Approximately one-third of schizophrenic patients receiving antipsychotic drugs obtain little or no benefit from them. In some reports, response has been linked with "prognostic" factors, including abnormal EEG and drug response (pharmaco-EEG). Measurements of topological EEG spectral power and Brief Psychiatric Rating Scale (BPRS) values were obtained in 15 schizophrenics over a period of 20 min immediately after the first application, and at weekly intervals after single intramuscular injection of 2 ml depot haloperidol (Haldol-Decanoate). A highly significant increase in the alpha-1 power density at almost all positions and an increase in theta power at parietotemporal regions can be described as electrophysiological parameters of the acute pharmaco-EEG. BPRS values decreased continuously within the first 2 weeks followed by an increase to the predrug level after the fourth week. Over the whole observation period there was a correlation between neurophysiological data (alpha-1 power) and clinical symptomatology (BPRS total score; r = -0.7). Therefore, it can be concluded that quantitative EEG recordings are able to serve as a tool for the optimization of the depot neuroleptic treatment in schizophrenics.