Catabolism of 5-hydroxytryptamine in the midbrain raphe nuclei of aggressive C57Bl/6J mice increased after 10 and 20 days of confrontations. Both catabolism and concentration of 5-hydroxytryptamine increased in the dopaminergic nuclei A11, A10, A9, and in the amygdala. The level of 5-hydroxyindoleacetic acid in the A9 and raphe nuclei decreased after 20 days of confrontations, which coincided with manifestation of the immune response.