Activity of enzymes catalyzing synthesis and degradation of serotonin and dopamine in brain structures of Wistar and August rats was measured biochemically under normal conditions and after short-term exposure to delta-sleep-inducing peptide. The effects of the test peptide manifested in activation of the serotoninergic system and inhibition of the dopaminergic system, particularly in the caudate nucleus. These changes were most pronounced in the brain of Wistar rats.