The augmentating-reducing response in visual evoked potentials (VEP) has been shown to be a useful tool in psychiatric research. A series of studies has been devoted to the search for the biochemical basis of this phenomenon. An augmenter response in VEP is shown to be clearly related to low levels of endorphins, 5-HIAA and homovanillic acid in CSF and to low serum-dopamine-β-hydroxylase activity, while no significant relationship was found with 4-hydroxy-3-methoxyphenylethylene glycol or tryptophan in CSF or MAO in thrombocytes. The results indicate that an augmenter response in VEP is related to low activity in the serotoninergic, dopaminergic and endophinergic pathways in CNS