In 57 consecutive patients with psychiatric disorders correlations were sought between the neurophysiological variables - amplitudes and latencies of visual averaged evoked responses (VAER), the proneness to augment or to reduce the amplitude of VAER with increasing stimulus intensity, PAR and PRI, the mean integrated amplitude (MIA) and its within patient variance (WPV) and the monoamine oxidase activity (MAO) in platelets, both with beta-fenylethylamine (MAO1) and with tryptamine (MAO2) as substrate. When multiple correlations with age occurred, a multiple regression was calculated to evaluate the relative importance of each component. MAO1 was found to be significantly correlated to Ampl. N1 and Lat. 2, 3 and 4 of VAER while MAO2 only gave a significant correlation with LaT. 4. As in earlier studies Ampl. N1 was found to be significantly correlated to the content of 5-hydroxyindoleacetic acid in liquor, these measurements seem to be the most important in psychiatric disorders where the monoaminergic systems are supposed to be disturbed.