Using a classical click/flash paradigm, the CNV was recorded from the following three groups of subjects at Broadmoor Hospital: 14 'psychopathic' patients selected by use of the 4/9 MMPI profile and confirmed by independent clinical diagnosis; 15 'non psychopathic' patients, all psychotic and mainly schizophrenic; 14 healthy staff control subjects. All three groups were matched for age and sex; the two patient groups were also matched for length of stay. Two series of 32 paired stimuli were used, separated by an interval of 30 minutes. The mean CNV voltage was significantly lower in the 'non-psychopathic' patients. The amplitude of the 'psychopath's' CNV response did not differ significantly from that of the staff controls, but the response variability between the first and second series of trials was much greater in the 'psychopathic' patients than in the other two subject groups. The 'psychopathic' subjects tended to show more rapid initial development of the CNV.