In patients suffering from chronic pain, visual evoked potentials (VEP) and pain measures (PM) were investigated before and after a 4-week, double-blind, controlled study of a new, rather selective, serotonin-reuptake inhibitor, Zimelidine, versus placebo. Before the trial most of the patients showed an augmenter response in VEP which means that the maximum amplitude of the evoked potential increased when stimulus intensity increased. After treatment most of the patients in the Zimelidine group revealed a reducer response, i.e. the maximum amplitude of the evoked potential decreased despite the increase of stimulus intensity, while no significant changes occurred in the placebo group. Only small and mostly insignificant changes were noted in PM. The results indicate that the augmenter-reducer response in VEP may be a useful measurement for following changes in the serotoninergic pathways in CNS. The results also indicate that treatment with Zimelidine may result in a decreased sensitivity to noxious stimulation that should be reflected in relief of clinical pain syndromes.