The influence of stimulation frequency and stimulus intensity on the auditory evoked potential components N1 and P2 was investigated in schizophrenic and major depressive patients. The findings in the patients were compared with those in normal controls. At a high stimulation frequency the amplitude of N1 was enhanced in both schizophrenic and major depressive patients; the latency of N1 increased only in the schizophrenic patients. These changes may be related to impairments of auditory input control and processing in these diseases. In the schizophrenic patients, P2 latency was prolonged under treatment with high-potency neuroleptic drugs.