Effects of the peptide drugs selank (Thr-Lys-Pro-Arg-Pro-Gly-Pro) and tuftsin (Thr-Lys-Pro-Arg) on the metabolism of serotonin (5-hydroxytryptamine, 5-HT) in the brain of Wistar rats preliminarily treated with the 5-HT-synthesis inhibitor p-chlorophenylalanine (PCPA, 320 mg/kg, i.p., 4 days before experiment) have been studied on a group of 87 matured rats. It is established that selank enhances the 5-HT metabolism in the brain stem 30 min after injection to animals pretreated with PCPA. In contrast to selank, tuftsin induced no changes in the 5-HT metabolism in the brain stem and decreased it in the neocortex on the same time scale. The data obtained suggest that the peptide drug selank can be used for the correction of disturbances induced by a decrease in 5-HT metabolism.