Abstract Diethyldithiocarbamate (DEDTC) given orally to rats without any addition of copper considerably increased the concentration of Cu in the brain without any change in the other tested tissues. Cysteine, comparatively studied, did not induce any change in the brain Cu level. Based on these findings and the literature data concerning DEDTC effects in animal and human, we put forward the hypothesis that the main effect of DEDTC is to provoke in the brain not a deficiency but an excess of Cu liberated from the lipophilic complex Cu-DEDTC. Cu is then engaged in an oscillatory oxido reduction giving a Cu ++ cation radical able to induce deleterious effects on tissues in a similar way as paraquat. The practical consequences of this hypothesis are considered.