Somatosensory, visual and auditory evoked potentials (EP) were recorded in different psychatric populations before as well as during psychotropic drug treatment. Drug-free schizophrenic patients showed shorter latencies, smaller amplitudes and an increased intraindividual variability in their EP than controls. Psychotic children but also children of schizophrenic mothers (so-called high-risk children) exhibited similar differences as compared to controls, suggesting a CNS overarousal as the pathoneurophysiological substrate of schizophrenia. Shorter latencies were also seen in children of psychopathic fathers. Regression and correlation analysis of psychopathological and EP measurements in hyperkinetic children revealed the following findings: the shorter the latencies and the higher the amplitudes, the sicker was the child. During psychopharmacotherapy, significant changes occurred in the EP measurements, which were found to be significantly correlated with clinical improvement or deterioration. Neuroleptics induced a latency increase and an amplitude decrease in schizophrenic patients and psychotic children. Interestingly, amphetamine produced in hyperkinetic children a latency increase too, thus explaining the 'paradoxical' clinical response to amphetamine observed in these children. Differences between therapy-responsive and therapy-resistant patients are described and, finally, some data concerning the role of the pretreatment EP as a predictor of therapeutic outcome are discussed.