The distribution of the glyoxylate dehydrogenase (GLYO-DH) (Glyoxylate oxidoreductase) activity has been investigated in rat brain using the corresponding histochemical method. The results have demonstrated a positive histochemical reaction in neurons of several cortical and subcortical areas. These neurons are located in the same regions in which operate monoaminergic neurotransmitters. These facts suggest a possible interaction between glyoxylic acid and monoaminergic neurotransmission.